Downhole communication

ABSTRACT

Downhole electrical energy harvesting and communication in systems for well installations having metallic structure carrying electric current, for example CP current. In some instances there is a harvesting module ( 4 ) electrically connected to the metallic structure ( 2 ) at a first location and to a second location spaced from the first location, the first and second locations being chosen such that, in use, there is a potential difference therebetween due to the electric current flowing in the structure ( 2 ); and the harvesting module ( 4 ) being arranged to harvest electrical energy from the electric current. In addition or alternatively, there may be communication apparatus ( 4, 5, 6 ) for communication by modulation of the current, for example CP current, in the metallic structure ( 2 ).

This invention relates to downhole communication. In a particular caseit relates to methods and systems for communicating using devices in awell installation having metallic structure provided with cathodicprotection. The invention also relates to methods and systemsincorporating energy harvesting methods and systems as well as apparatusfor use in such methods and systems.

There is a general desire to be able to extract data from oil and/or gaswells as well as control devices in oil and/or gas wells such asvalves—say for example sub-surface safety valves.

However, providing power to such downhole devices represents achallenge. There are some circumstances where power may be provideddirectly from the surface via a cable or devices may be powered directlyfrom the surface using hydraulic power. However, in other circumstancesthese methods of power delivery are not appropriate. In somecircumstances the use of batteries becomes an option. However, this initself represents challenges particularly in the downhole environmentwhere the relatively high temperatures tend to lead to shortened batterylife.

Therefore it is desirable to provide alternative sources of poweringdownhole devices which can be used in circumstances where the deliveryof power directly from the surface via a cable or hydraulically isdifficult, impossible or undesirable whilst avoiding the limitationswhich are encountered if battery power is relied upon. It is alsodesirable to provide alternative methods for communicating betweendownhole locations and other downhole and/or surface locations.

In the present specification the expression surface encompasses the landsurface in a land well where a well head will be located, theseabed/mudline in a subsea well, and a well head deck on a platform. Italso encompasses locations above these locations where appropriate.Generally “surface” is used to refer to any convenient location forapplying and/or picking up power/signals for example, which is outsideof the borehole of the well.

According to a first aspect of the invention there is provided adownhole electrical energy harvesting system for harvesting electricalenergy in a well installation having metallic structure carryingelectric current, the system comprising:

a harvesting module electrically connected to the metallic structure ata first location and to a second location spaced from the firstlocation, the first and second locations being chosen such that, in use,there is a potential difference therebetween due to the electric currentflowing in the structure; and the harvesting module being arranged toharvest electrical energy from the electric current.

The well installation may be one with cathodic protection such that theelectric current is cathodic protection current. Whilst the presenttechniques could be used in a system where current is specificallyapplied to the downhole structure for use in power delivery, it has beenrealised that it is possible to harvest power from cathodic protectionsystems and that is particularly preferred if the power can be harvestedfrom currents which are already present.

The second location will generally be a downhole location.

In some instances the connection to the second location may be aconnection to the formation via an electrode. Most typically however,the harvesting module will be connected to the metallic structure at thefirst and second spaced locations.

Such systems and methods are advantageous because power may be providedto a downhole device without having to provide a separate power supply.Moreover the power may be supplied without having to rely on localbatteries which will tend to have a limited life and may be suppliedwithout having to provide a cable which penetrates through the wellhead. Similarly these techniques may be implemented without usingtoroids to inject or extract signals. This reduces the complexity andtechnical issues which will be incurred in implementing a system.

The harvesting module may be arranged to harvest electrical energy fromdc currents.

Preferably the current flow within portions of the metallic structure inregions between the first location and second location is in the samelongitudinal direction.

Preferably there is an uninterrupted current flow path between the firstlocation and the second location which is at least partly via themetallic structure.

These represent features which will generally be present in aninstallation unless modification is made to the set up. The presentideas generally do not need modifications to the standard set up of thewell installation as a whole, that is they are aimed at workingalongside a standard installation.

The harvesting module may be electrically connected to the metallicstructure at the second location.

The or each connection to the metallic structure may be made to a run ofmetallic elongate members/a run of metallic pipe.

In one set of embodiments the spaced locations may be axially spaced.The connections may be made to a common run of metallic elongatemembers, for example a common run of metallic pipe which is part of themetallic structure. The uppermost of the two spaced locations may beadjacent to the location of a liner hanger provided in the well. Oftenthis will represent the highest practical location for the uppermostlocation. In some instances the upper connection may be made to a riser.

Thus, for example the connections may both be made to production tubingprovided in the well, or both made to a first run of casing separated bya first, “A”, annulus from the production tubing, or both made to asecond run of casing separated by a second, “B”, annulus from the firstrun of casing, or so on.

In other cases, axially spaced connections may be made to different runsof metallic elongate members, for example different runs of metallicpipe with similar results, but it is generally more convenient to makethe connections to the same run of metallic elongate members/metallicpipe if there is no reason to do differently.

Where the spaced locations are axially spaced and this is relied uponfor there to be a potential difference therebetween, the spacing betweenthe locations is likely to be considerable—typically 100 m or more. Morepreferably 300 m to 500 m.

The electrical connection to the metallic structure at the firstlocation may be a galvanic connection.

The electrical connection to the metallic structure at the secondlocation may be a galvanic connection.

The harvesting module may be positioned in one or more of external tothe well elongate members, within an annulus of the well, and within aninternal bore of the well.

The connection to at least one of the first and second locations may bevia a cable running alongside the metallic structure.

Preferably if the second spaced contact is made to the at least one runof metallic elongate members then the electrical current flowing in theat least one run of metallic elongate members where the first contact ismade flows in the same longitudinal direction as the electrical currentflowing in the at least one run of metallic elongate members where thesecond contact is made.

Preferably if the first spaced contact and the second spaced contact areboth made to the same run of metallic elongate members, that run ofmetallic elongate members is continuously conductive between the firstand second locations.

At least one connection between the at least one of the electricalcontacts and the harvesting module may be provided by an insulatedcable.

The cable may be selected to have a conductor with a relatively largecross-sectional area. When selecting a cable the aim is to pick across-sectional area which is large enough to allow the desired level ofharvesting—one which provides low enough resistance in the cable.

Preferably the insulated cable has a conductive area of at least 10 mm̂2,preferably at least 20 mm̂2, more preferably at least 80 mm̂2.

The cable may be a tubing encapsulated conductor.

One of the connections may be made without an external cable. One of theconnections may be made via a conductive housing of or surrounding theharvesting module.

Typically there will be an optimal spacing between the connections. Thelarger the spacing the greater the change in potential between thecontact locations, but also the greater the resistance of the cable. Themethod may comprise determining an optimal spacing, between the spacedlocations. This may be determined by modelling for a particularinstallation.

The spacing between the locations may be at least 100 m.

In another set of embodiments the spaced locations may be radiallyspaced. A first of the connections may be made to a first run ofmetallic elongate members, for example a first run metallic pipe whichis part of the metallic structure and a second of the connections may bemade to a second, distinct, run of metallic elongate members, forexample, a second, distinct, run of metallic pipe which is part of themetallic structure. Thus the connection may be across an annulus definedby two runs of metallic pipe.

For example, one connection may be made to production tubing provided inthe well and one to a first run of casing separated by a first, “A”,annulus from the production tubing, or one connection may be made to afirst run of casing provided in the well and one to a second run ofcasing separated by a second, “B”, annulus from the first run of casing,and so on.

In some cases the spaced locations may be both axially spaced andradially spaced.

The connections may be made to a common run of metallic elongate memberswhich is part of the metallic structure.

In some embodiments a first of the connections is made to a first run ofmetallic elongate members which is part of the metallic structure and asecond of the connections is made to a second, distinct, run of metallicelongate members which is part of the metallic structure.

Insulation means may be provided for electrically insulating the firstrun of metallic elongate members from the second run of metallicelongate members in the region of the connections.

Insulation means may be provided for electrically insulating the firstrun of elongate members/metallic pipe from the second run of elongatemembers/metallic pipe in the region of at least one of the connections.This can help ensure that there is a potential difference between theruns of elongate members/metallic pipe at the locations where theconnections are made. This being due to the different path to earth seenfrom each run of members/pipe.

Note that in the present techniques the currents from which energy isharvested will generally be flowing in the same direction in the firstand second runs of metallic elongate members/pipe. Thus the insulationis not provided to form a separate return path but rather to alter thepath to earth for one of the runs relative to the other.

The insulation means may comprise an insulation layer or coatingprovided on at least one of the runs of elongate members/metallic pipe.The insulation means may comprise at least one insulating centraliserfor holding the runs of elongate members/metallic pipe apart from oneanother.

The insulation means may be provided to avoid electrical contact betweenthe two runs of elongate members/metallic pipe for a distance of atleast 100 m, preferably at least 300 m.

At least one of the connections may be located within the insulatedregion. Both of the connections may be located within the insulatedregion. At least one of the connections may be located towards amidpoint of the insulated region. The location of at least one of theconnections may be determined by modelling of a particular installationto determine an optimum location which is then selected.

The harvesting module may be provided in the bore of a central run oftubing, in an annulus or outside the casing—between the casing and theformation. Thus amongst, other possible locations, the harvesting modulemay be provided in the “A” annulus, the “B” annulus, the “C” annulus,the “D” annulus, or any further annulus.

This gives rise to the possibility of providing power in locations whereit is generally not possible and/or desirable to provide cables from thesurface. This is particularly useful for subsea wells. Further this ispossible without relying on the use of primary batteries or anotherlocal power source, and thus there is a possibility of providing “lifeof well” power in such locations.

The harvesting module may comprise variable impedance means for varyingthe load seen between the two connections. The variable impedance meansmay be microprocessor controlled.

The variable impedance means may be used to vary the load so as tooptimise energy harvesting.

The variable impedance means may be used to modulate the load so as tocommunicate data from the harvesting module towards the surface.

Downhole communication means may be provided for transmitting data fromdownhole towards the surface. The downhole communication means may alsobe arranged for receiving data, for example from the surface.

The harvesting module may comprise downhole communication means. Inother cases the downhole communication means may be provided separately.A downhole device which is powered by the harvesting module may comprisethe downhole communication means.

The downhole communication means may comprise the variable impedancemeans.

Upper communication means may be provided at an out of bore holelocation including a detector for detecting changes in the current, saythe cathodic protection current, flowing in the metallic structure andhence allowing extraction of data encoded by modulation of the load atthe harvesting module. For example the detector may be arranged todetect the potential of the metallic structure relative to a referenceor to detect the potential seen across; or current seen by, a powersupply used to apply an impressed cathodic protection current to themetallic structure.

In other embodiments rather than communicating towards the surface bymodulating the load other communication techniques may be used. Ingeneral, for example, acoustic and/or EM (Electro-Magnetic) signallingmay be used. Modulating the load is one example of EM signalling, butother, more direct means of EM signalling may be used.

The downhole communication means may be arranged to apply acoustic datacarrying signals to the metallic structure and the upper communicationmeans may be arranged to receive acoustic data carrying signals.

The downhole communication means may be arranged to apply EM(Electro-Magnetic) data carrying signals to the metallic structure andthe upper communication means may be arranged to receive EM datacarrying signals.

The upper communication means may be arranged to apply acoustic and/orEM (Electro-Magnetic) data carrying signals to the metallic structure,and the downhole communication means may be arranged to receive acousticand/or EM data carrying signals.

In some cases the upper communication means and the downholecommunication means may be arranged to communicate using both acousticand EM signals. This creates useful redundancy in that if onecommunication channel fails the other may remain operational.

The harvesting module may be disposed at a selected location downholefor harvesting power and a cable may be provided for supplyingelectrical power further downhole to a downhole device. The crosssectional area of the cable used to supply the electrical power furtherdownhole will typically be smaller than that of any cable used inharvesting the power, and typically the power will be supplied furtherdownhole at a higher voltage than the voltage developed across thespaced contacts due to current flowing in the metallic structure, duefor example to cathodic protection currents.

In some embodiments the current flowing in the elongate members issupplied from the surface of the well.

In some embodiments the current flowing in the elongate member issupplied from one or more sacrificial anodes.

In some embodiments the current flowing in the elongate members is animpressed current from an external power supply.

In some embodiments the voltage of the surface of the well is, in use,limited to the range minus 0.7 volts to minus 2 volts with respect to asilver/silver chloride reference cell.

Preferably the potential difference between the spaced contacts is lessthan 1 volt, preferably less than 0.5 volts, more preferably less than0.1 volts.

Optionally the resistance of the well structure between the contacts isless than 0.1 ohms, preferably less than 0.01 ohms.

The optimal location for harvesting power will typically be near to thelocation at which the currents, for example, the cathodic protectioncurrents are injected into the metallic structure.

Where the spaced locations are spaced axially, preferably the upperlocation is adjacent the location at which the currents, for example,the cathodic protection currents are injected into the metallicstructure. Note that where there is a platform structure, the current,for example, the cathodic protection currents may reach the downholemetallic structure via a galvanic connection to the platform structure.In some cases the present techniques may include controlling thelocation of that connection.

The optimal location for harvesting power will often be near to the wellhead where there is the greatest rate of change in potential as oneprogresses down into the well. On the other hand a downhole device to bepowered may be further downhole. Thus the harvesting module and downholedevice may be at different locations, in particular, different depths inthe well.

In other situations, the harvesting module and downhole device may belocated together. The system may comprise a downhole unit whichcomprises the harvesting module and the downhole device.

The upper spaced contact may be:

where the well is a land well, within 100 m, preferably within 50 m ofthe land surface; and

where the well is a subsea well, within 100 m, preferably within 50 m ofthe mudline.

The upper spaced contact may be located adjacent to a location whichcorresponds to a maxima in magnitude of potential caused by the electriccurrent flowing in the structure.

The system may further comprise downhole communication means fortransmitting and/or receiving data.

The downhole communication means may be arranged for transmitting databy varying the load seen between the connections at the spacedlocations.

According to another aspect of the invention there is provided adownhole device operation system comprising a downhole electrical energyharvesting system as defined above and a downhole device, the harvestingmodule being electrically connected to and arranged for providing powerto the downhole device.

The downhole device may comprise a downhole sensor for example apressure and/or temperature sensor. The sensor may be installed, forexample, in the “A”, “B”, “C” or “D” annulus.

A sensor disposed in one annulus or bore may be arranged to monitor aparameter in an adjacent annulus or bore as well as or instead of in theannulus or bore in which it is located. A port may be provided through arun of metallic structure to allow sensing in an adjacent annulus orbore.

A sensor may be provided for detecting a leak in a cemented annulus.

A sensor may comprise an array of sensors.

The downhole device may comprise at least one of:

a downhole sensor;

a downhole actuator;

an annular sealing device, for example a packer, or a packer element;

a valve;

a downhole communication module, for example a transceiver or repeater.

The communication module may comprise a downhole communicationsrepeater. This may be a repeater for acoustic communication, or EMcommunication including wireless EM communication and cable borne EMcommunication, or for a hybrid communication system. For example, therepeater may receive acoustic signals from further downhole and signaltowards the surface using EM communication or vice versa. Similarly bothacoustic and EM communication may be used in one or both directions. EMsignalling may be achieved by applying electrical signals downhole ormodulating the load in the harvesting module as described above. EMsignalling may be at least partly along cables as mentioned above.

Where the downhole device is a repeater or a transceiver, the system maybe pre-installed in a well installation to make the well “wirelessready”. That is, the system may be installed to provide a wirelesscommunication backbone even though the communication ability may not beused initially. Here again wireless refers to there being at least onewireless leg in the communication channel, other legs may be via cable.

In other situations the system may be retro-fitted.

The valve may comprise at least one of:

a subsurface safety valve;

a bore flow control valve;

a bore to annulus valve;

an annulus to annulus valve;

a bore to pressure compensation chamber valve;

an annulus to pressure compensation chamber valve;

a through packer or packer bypass valve.

Note that each device may be a remotely controlled device which may be awirelessly controlled device, for example in the sense that wherecontrolled from the surface there is at least one wireless leg in thecommunications channel. Other legs may be via cable e.g. between asensor location and the harvesting location.

EM signalling may be using dc or ac signals and appropriate modulationschemes. The harvesting module may comprise a dc to dc convertor forharvesting power from the cathodic protection currents or other currentpresent. The harvesting module may comprise an energy storage device forstoring harvested power. The energy storage device may comprise a chargestorage device which may comprise at least one capacitor and/or at leastone re-chargeable battery. Where there is energy storage means, theharvesting module may be arranged to selectively supply power from thestorage device or directly from harvested energy. This selection may bemade based on predetermined conditions. Alternatively there may be noenergy storage device and the harvesting module may be arranged tosupply power continuously when required.

A primary battery may also be provided at the harvesting module forselective use.

The dc to dc converter may comprise a Field Effect Transistor arrangedto form a resonant step-up oscillator. The dc to dc convertor mayinclude a step-up transformer and may include a coupling capacitor.

The harvesting module may be arranged to control the turns ratio of thestep-up transformer to modify the load generated by the dc-dc converter.A secondary winding of the step-up transformer may comprise a pluralityof tappings and/or the step-up transformer may comprise a plurality ofsecondary windings and the harvesting module may be arranged to selectwindings and/or tappings to provide a desired turns ratio. Amicroprocessor controlled switch may be used to select tappings and/orwindings.

According to another aspect there is provided a downhole unit comprisinga harvesting module as defined above and at least one device arranged tobe powered by the harvesting module.

One or more of the sensor module, the communication module, and theharvesting module may be provided in an annulus—for example the “B”annulus or the “C” annulus or another annulus. The sensor module and theharvesting module may be provided as part of a common downhole unit,however more typically they will be separate so that the sensor may belocated deeper than the harvesting module.

The downhole device may be provided at a different location in the wellthan the harvesting module.

The harvesting module may be disposed at a selected location downholefor harvesting power and a cable may be provided for supplyingelectrical power further downhole to the downhole device at a differentlocation in the well.

The cross sectional area of the conductive core, or cores, of the cableused to supply the electrical power further downhole may be smaller thanthat of cable used to connect the harvesting module to the downholestructure for harvesting the power.

According to another aspect of the invention there is provided adownhole well monitoring system for monitoring at least one parameter ina well installation having metallic structure carrying electric current,the system comprising:

an electrical energy harvesting system as defined above;

a sensor module for sensing at least one parameter; and

a communication module for sending data encoding readings from thesensor module towards the surface,

the electrical energy harvesting system being arranged to supplyelectrical power to at least one of the sensor module and thecommunications module.

According to another aspect of the invention there is provided adownhole well monitoring system for monitoring at least one parameter ina well installation having metallic structure carrying electric current,the system comprising:

a sensor module for sensing at least one parameter;

a communication module for sending data encoding readings from thesensor module towards the surface; and

an electrical energy harvesting system comprising a harvesting moduleelectrically connected to the metallic structure at a first location andto a second location spaced from the first location, the first andsecond locations being chosen such that, in use, there is a potentialdifference therebetween due to the electric current flowing in thestructure; and the harvesting module being arranged to harvestelectrical energy from the electric current, the electrical energyharvesting system being arranged to supply electrical power to at leastone of the sensor module and the communications module.

The system may comprise at least one first length of cable forconnecting the harvesting module to one of the spaced locations.

The system may comprise at least one second length of cable forsupplying power from the harvesting module to the sensor module.

The cross-sectional area of the conducting portion of the first lengthof cable may be greater than the cross-sectional area of the conductingportion of the second length of cable.

The communication module may be arranged for modulating the electriccurrent flowing in the metallic structure at a signalling location so asto encode data to allow extraction of the data at a reception locationremote from the signalling location by detection of the effect of saidmodulation on the electric current at said reception location.

The well monitoring system may comprise a detector for detecting theeffect of said modulation on the electric current at said receptionlocation to extract the encoded data.

The communication module may be arranged for controlling the loadgenerated by the harvesting module to cause said modulation of theelectric current in the metallic structure at the signalling location.

The sensor module may comprise a pressure sensor.

The pressure sensor may be arranged for monitoring the reservoirpressure of the well.

The pressure sensor may be arranged for monitoring the pressure in anannulus of the well.

The pressure sensor may be arranged for monitoring the pressure in anenclosed annulus of the well.

According to another aspect of the invention there is provided adownhole communication repeater system for use in a well installationhaving metallic structure carrying electric current, the systemcomprising:

an electrical energy harvesting system as defined above; and

a communications repeater disposed downhole in the well and arranged forcommunicating with a first device beyond the well head using acommunication channel which is wireless at least through the well headand arranged for communicating with second device located in the welland thus below the well head such that the communications repeater mayact as a repeater between the first and second devices,

the electrical energy harvesting system being arranged to supplyelectrical power to communications repeater.

According to another aspect of the invention there is provided adownhole communication repeater system for use in a well installationhaving metallic structure carrying electric current, the systemcomprising:

a communications repeater disposed downhole in the well and arranged forcommunicating with a first device beyond the well head using acommunication channel which is wireless at least through the well headand arranged for communicating with second device located in the welland thus below the well head such that the communications repeater mayact as a repeater between the first and second devices; and

an electrical energy harvesting system comprising a harvesting moduleelectrically connected to the metallic structure at a first location andto a second location spaced from the first location, the first andsecond locations being chosen such that, in use, there is a potentialdifference therebetween due to the electric current flowing in thestructure; and the harvesting module being arranged to harvestelectrical energy from the electric current, the electrical energyharvesting system being arranged to supply electrical power tocommunications repeater.

It will be appreciated that here reference to a first device beyond thewell head refers to one on the other side of the well head than thesecond device which is in the well such that communication across thewell head is desired. Ultimately, the first device could be locatedalmost anywhere, be that close to the well head or at a remote location,provided that appropriate communications are provided.

The communications repeater may be arranged for modulating the electriccurrent flowing in the metallic structure at a signalling location so asto encode data to allow extraction of the data at a reception locationremote from the signalling location by detection of the effect of saidmodulation on the electric current at said reception location.

The communications repeater and/or the harvesting module may be providedin an annulus—for example the “B” annulus or the “C” annulus or anotherannulus.

The communications repeater and the harvesting module may be provided aspart of a common downhole unit.

The system may comprise at least one first length of cable forconnecting the harvesting module to one of the spaced locations.

The system may comprise at least one second length of cable forsupplying power from the harvesting module to the communicationsrepeater.

The cross-sectional area of the conducting portion of the first lengthof cable may be greater than the cross-sectional area of the conductingportion of the second length of cable.

The downhole communication repeater system may comprise a detector fordetecting the effect of said modulation on the electric current at saidreception location to extract the encoded data.

The communications repeater may be arranged for controlling the loadgenerated by the harvesting module to cause said modulation of theelectric current in the metallic structure at the signalling location.

According to another aspect of the present invention there is provided adownhole device operation system for operating a downhole device in awell installation having metallic structure carrying electric current,the system comprising:

a downhole device;

an electrical energy harvesting system comprising a harvesting moduleelectrically connected to the metallic structure at a first location andto a second location spaced from the first location, the first andsecond locations being chosen such that, in use, there is a potentialdifference therebetween due to the electric current flowing in thestructure; and the harvesting module being arranged to harvestelectrical energy from the electric current, the electrical energyharvesting system being arranged to supply electrical power to thedownhole device.

The downhole device may comprise at least one of:

a downhole sensor;

a downhole actuator;

an annular sealing device, for example a packer, or a packer element;

a valve;

a downhole communication module, for example a transceiver or repeater.

The valve may comprise at least one of:

a subsurface safety valve;

a bore flow control valve;

a bore to annulus valve;

an annulus to annulus valve;

a bore to pressure compensation chamber valve;

an annulus to pressure compensation chamber valve;

a through packer or packer bypass valve.

The power may be supplied to control the valve, with power for movingthe valve coming from another source (e.g. spring loading, differentialpressure), or supplied for moving the valve or for control and moving ofthe valve. The valve may comprise a trigger mechanism for example apilot valve that is operated using power from the power delivery system.

The device operating system may be arranged to supply variable powerlevels. Thus a first power level may be provided other than at timeswhen a second higher power level is required. The applied currents, forexample the cathodic protection currents may be increased when thehigher power level is required by switching in more anodes or applying ahigher impressed current.

This might be at a level which is undesirable long term due to thepotentially damaging effects of too high a potential difference causedby the cathodic protection currents—hydrogen embrittlement—butacceptable short term. Thus the system, apparatus, method may bearranged for temporarily increasing the applied current, for example thecathodic protections current. The higher power level may be used forexample to move a valve from one state to another, with the lower levelused at other times, for example monitoring and/or control signals.

The downhole device may be provided at a different location in the wellthan the harvesting module.

The harvesting module may be disposed at a selected location downholefor harvesting power and a cable may be provided for supplyingelectrical power further downhole to the downhole device at a differentlocation in the well.

The cross sectional area of the conductive core, or cores, of the cableused to supply the electrical power further downhole may be smaller thanthat of cable used to connect the harvesting module to the downholestructure for harvesting the power.

A further source of power may be available to the downhole devicebesides electrical power supplied by the electrical energy harvestingmodule.

In each of the above apparatus, the harvesting module may comprisevariable impedance means for varying the load seen between the twoconnections. The variable impedance means may be microprocessorcontrolled.

The variable impedance means may be used to vary the load so as tooptimise energy harvesting.

The variable impedance means may be used to modulate the load so as tocommunicate data from the harvesting module towards the surface.

Impedance modulation may also be used in communicating from an upperlocation towards the harvesting module so as to modulate the applied(e.g. cathodic protection) current. One possibility is to switch ananode into and out of operation which will modulate the potential seendownhole. Thus data may be encoded by switching the anode into and outof operation. For example the connection between the anode and thestructure may be selectively made and broken with switch means. Thus theupper communication unit may comprise a switch means for switching ananode into and out of operation. In an impressed current system theapplied signals may be modulated to encode data.

According to another aspect of the present invention there is provided amethod of powering a downhole device in a well installation havingmetallic structure carrying electric current, the method comprising thesteps of: electrically connecting a harvesting unit to the metallicstructure at a first location and to a second location spaced from thefirst location, the first and second locations being chosen such thatthere is a potential difference therebetween due to the electric currentflowing in the structure and the harvesting unit being arranged toharvest electrical energy from electric current when connected betweenlocations having a potential difference therebetween;

harvesting electrical power from the electric current at the harvestingunit; and supplying electrical power from the harvesting unit to thedownhole device.

The method may comprise the steps of: determining a location where thereis a maxima in magnitude of potential caused by the electric currentflowing in the structure, and choosing the first location, where theharvesting unit is connected to the metallic structure, in dependence onthe location of said maxima.

According to another aspect of the present invention there is provided adownhole electrical energy harvesting system for use in a wellinstallation having metallic structure comprising at least one run ofmetallic elongate members carrying electrical current, the harvestingsystem comprising: an energy harvesting module comprising an electricalcircuit connected between spaced contacts to harvest energy from apotential difference between the spaced contacts, wherein a first of thespaced contacts is made to the at least one run of metallic elongatemembers at a first location and a second of the spaced contacts is madeto the at least one run of metallic elongate members at a secondlocation and the potential difference is caused by the current flowingin the at least one run of elongate members and, at least in part, theimpedance of the at least one run of elongate members.

The electrical current flowing in the at least one run of metallicelongate members where the first contact is made may flow in the samelongitudinal direction as the electrical current flowing in the at leastone run of metallic elongate members where the second contact is made.

Preferably if the first spaced contact and the second spaced contact areboth made to the same run of metallic elongate members, that run ofmetallic elongate members is continuously conductive between the firstand second locations.

Preferably the metallic structure provides an uninterrupted current flowpath between the first location and the second location.

Preferably the current flow within portions of the metallic structure inregions between the first location and second location is in the samelongitudinal direction.

Preferably the harvesting module is arranged to harvest electricalenergy from dc currents.

The electrical connection to the metallic structure at the firstlocation may be a galvanic connection.

The electrical connection to the metallic structure at the secondlocation may be a galvanic connection.

The electrical connection to the metallic structure at the firstlocation may be made to one of: casing, liner, tubing, coiled tubing,sucker rod.

The electrical connection to the metallic structure at the secondlocation may be made to one of: casing, liner, tubing, coiled tubing,sucker rod.

The spaced locations may be axially spaced.

The spaced locations may be radially spaced.

At least one connection between the at least one of the electricalcontacts and the electrical circuit may be provided by an insulatedcable.

Preferably the insulated cable has a conductive area of at least 10 mm̂2,preferably at least 20 mm̂2, more preferably at least 80 mm̂2.

The cable may be a tubing encapsulated conductor.

The spacing between the locations may be at least 100 m.

The connections may be made to a common run of metallic elongate memberswhich is part of the metallic structure.

In some embodiments a first of the connections is made to a first run ofmetallic elongate members which is part of the metallic structure and asecond of the connections is made to a second, distinct, run of metallicelongate members which is part of the metallic structure.

Insulation means may be provided for electrically insulating the firstrun of metallic elongate members from the second run of metallicelongate members in the region of the connections.

The insulation means may comprise an insulation layer or coatingprovided on at least one of the runs of metallic elongate members.

The insulation means may comprise at least one insulating centraliserfor holding the runs of metallic elongate members apart from oneanother.

The insulation means may be provided to avoid electrical contact betweenthe two runs of metallic elongate members for a distance of at least 100m.

The current flowing in the elongate members may be supplied from thesurface of the well.

The current flowing in the elongate member may be supplied from one ormore sacrificial anodes.

The current flowing in the elongate members may be an impressed currentfrom an external power supply.

The voltage of the surface of the well may be, in use, limited to therange minus 0.7 volts to minus 2 volts with respect to a silver/silverchloride reference cell.

The potential difference between the spaced contacts may be less than 1volt, preferably less than 0.5 volts, more preferably less than 0.1volts.

The resistance of the well structure between the contacts may be lessthan 0.1 ohms, preferably less than 0.01 ohms.

The upper spaced contact may be:

where the well is a land well, within 100 m, preferably within 50 m ofthe land surface; and

where the well is a subsea well, within 100 m, preferably within 50 m ofthe mudline.

The upper spaced contact may be located adjacent to a location whichcorresponds to a maxima in magnitude of potential caused by the electriccurrent flowing in the structure.

The system may comprise downhole communication means for transmittingand/or receiving data.

The downhole communication means may be arranged for transmitting databy varying the load seen between the connections at the spacedlocations.

According to another aspect of the invention there is provided adownhole device operation system comprising a downhole electrical energyharvesting system defined above and a downhole device, the harvestingmodule being electrically connected to and arranged for providing powerto the downhole device.

The downhole device may comprise at least one of:

a downhole sensor;

a downhole actuator;

an annular sealing device, for example a packer, or a packer element;

a valve;

a downhole communication module, for example a transceiver or repeater.

The valve may comprise at least one of:

a subsurface safety valve;

a bore flow control valve;

a bore to annulus valve;

an annulus to annulus valve;

a bore to pressure compensation chamber valve;

an annulus to pressure compensation chamber valve;

a through packer or packer bypass valve.

The downhole device may be provided at a different location in the wellthan the harvesting module.

The harvesting module may be disposed at a selected location downholefor harvesting power and a cable may be provided for supplyingelectrical power further downhole to the downhole device at a differentlocation in the well.

The cross sectional area of the conductive core, or cores, of the cableused to supply the electrical power further downhole may be smaller thanthat of cable used to connect the harvesting module to the downholestructure for harvesting the power.

According to another aspect of the present invention there is provided amethod of powering a downhole device in a well installation havingmetallic structure carrying electric current, the method comprising thesteps of:

electrically connecting a harvesting unit to the metallic structure at afirst location and to the metallic structure at a second location spacedfrom the first location, the first and second locations being chosensuch that there is a potential difference therebetween due to theelectric current flowing in the structure and the harvesting unit beingarranged to harvest electrical energy from electric current whenconnected between locations having a potential difference therebetween;

harvesting electrical power from the electric current at the harvestingunit; and supplying electrical power from the harvesting unit to thedownhole device.

The method may comprise the further steps of: determining a locationwhere there is a maxima in magnitude of potential caused by the electriccurrent flowing in the structure, and choosing the first location, wherethe harvesting unit is connected to the metallic structure, independence on the location of said maxima.

According to another aspect of the present invention there is provided adownhole electrical energy harvesting system for harvesting electricalenergy in a well installation having metallic structure provided withcathodic protection, the system comprising:

a harvesting module electrically connected to the metallic structure ata first location and to a second location spaced from the firstlocation, the first and second locations being chosen such that, in use,there is a potential difference therebetween due to the cathodicprotection currents flowing in the structure; and

the harvesting module being arranged to harvest electrical energy fromthe cathodic protection currents.

The harvesting module may be arranged to harvest electrical energy fromdc currents.

The current flow within portions of the metallic structure in regionsbetween the first location and second location may be in the samelongitudinal direction.

There may be an uninterrupted current flow path between the firstlocation and the second location which is at least partly via themetallic structure.

The harvesting module may be electrically connected to the metallicstructure at the second location.

The spaced locations may be axially spaced.

The spaced locations may be radially spaced.

At least one connection between the at least one of the electricalcontacts and the harvesting module may be provided by an insulatedcable.

The insulated cable may be a conductive area of at least 10 mm̂2,preferably at least 20 mm̂2, more preferably at least 80 mm̂2.

The cable may be a tubing encapsulated conductor.

The spacing between the locations may be at least 100 m.

The connections may be made to a common run of metallic elongate memberswhich is part of the metallic structure.

A first of the connections may be made to a first run of metallicelongate members which is part of the metallic structure and a second ofthe connections may be made to a second, distinct, run of metallicelongate members which is part of the metallic structure.

Insulation means may be provided for electrically insulating the firstrun of metallic elongate members from the second run of metallicelongate members in the region of the connections.

The insulation means may comprise an insulation layer or coatingprovided on at least one of the runs of metallic elongate members.

The insulation means may comprise at least one insulating centraliserfor holding the runs of metallic elongate members apart from oneanother.

The insulation means may be provided to avoid electrical contact betweenthe two runs of metallic elongate members for a distance of at least 100m.

The current flowing in the elongate members may be supplied from thesurface of the well.

The current flowing in the elongate member may be supplied from one ormore sacrificial anodes.

The current flowing in the elongate members may be an impressed currentfrom an external power supply.

The voltage of the surface of the well may be, in use, limited to therange minus 0.7 volts to minus 2 volts with respect to a silver/silverchloride reference cell.

The potential difference between the spaced contacts may be less than 1volt, preferably less than 0.5 volts, more preferably less than 0.1volts.

The resistance of the well structure between the contacts may be lessthan 0.1 ohms, preferably less than 0.01 ohms.

The upper spaced contact may be:

where the well is a land well, within 100 m, preferably within 50 m ofthe land surface; and

where the well is a subsea well, within 100 m, preferably within 50 m ofthe mudline.

The upper spaced contact may be located adjacent to a location whichcorresponds to a maxima in magnitude of potential caused by the electriccurrent flowing in the structure.

The system may further comprise downhole communication means fortransmitting and/or receiving data.

The downhole communication means may be arranged for transmitting databy varying the load seen between the connections at the spacedlocations.

According to another aspect of the present invention there is provided adownhole device operation system comprising a downhole electrical energyharvesting system as defined above and a downhole device, the harvestingmodule being electrically connected to and arranged for providing powerto the downhole device.

The downhole device may comprise at least one of:

a downhole sensor;

a downhole actuator;

an annular sealing device, for example a packer, or a packer element;

a valve;

a downhole communication module, for example a transceiver or repeater.

The valve may comprise at least one of:

a subsurface safety valve;

a bore flow control valve;

a bore to annulus valve;

an annulus to annulus valve;

a bore to pressure compensation chamber valve;

an annulus to pressure compensation chamber valve;

a through packer or packer bypass valve.

The downhole device may be provided at a different location in the wellthan the harvesting module.

The harvesting module may be disposed at a selected location downholefor harvesting power and a cable may be provided for supplyingelectrical power further downhole to the downhole device at a differentlocation in the well.

The cross sectional area of the conductive core, or cores, of the cableused to supply the electrical power further downhole may be smaller thanthat of cable used to connect the harvesting module to the downholestructure for harvesting the power.

According to another aspect of the present invention there is provideddownhole data communication apparatus for use in a well installationhaving metallic structure provided with a cathodic protection systemsuch that there is an electrical circuit comprising the metallicstructure and an earth return around which an electrical current flowsas a result of the cathodic protection system, the downhole datacommunication apparatus comprising:

a first communication module for location at a first location andcomprising modulation means for modulating the electrical current at afirst location so as to encode data; and

a second communication module for location at a second location, spacedfrom the first location, and comprising a detector for detecting theeffect of the modulation of the electrical current at the first locationso as to extract said data.

The modulation means may be arranged to at least one of:

i) where the cathodic protection system is an impressed cathodicprotection system, control a signal source of the impressed cathodicprotection system to directly modulate the cathodic protection currentapplied to the metallic structure;

ii) modify the connection between at least one anode of the cathodicprotection system and the metallic structure; and

iii) alter the impedance of the electrical circuit.

The first communication module may be arranged for location downhole.

The second communication module may be arranged for location downhole.

The apparatus may comprise a sensor module for sensing at least oneparameter, wherein the first communication module is arranged forsending data encoding readings from the sensor module towards the secondcommunication module.

The sensor module may comprise a pressure sensor.

The second communication module may be arranged for providing data to adownhole device in dependence on data received by the secondcommunication module from the first communication module.

The downhole device may comprise at least one of:

a downhole sensor;

a downhole actuator;

an annular sealing device, for example a packer, or a packer element;

a valve;

a downhole communication module, for example a transceiver or repeater.

The valve may comprise at least one of:

a subsurface safety valve;

a bore flow control valve;

a bore to annulus valve;

an annulus to annulus valve;

a bore to pressure compensation chamber valve;

an annulus to pressure compensation chamber valve;

a through packer or packer bypass valve.

At least one of the first and second communication modules may comprisea communications repeater for location downhole in a well and arrangedfor communicating with a first device beyond the well head using acommunication channel which is wireless at least through the well headand arranged for communicating with second device located in the welland thus below the well head such that the communications repeater mayact as a repeater between the first and second devices.

The apparatus may comprise a downhole electrical power harvesting modulearranged for electrical connection between two spaced locations in awell installation and comprising an electrical circuit arranged forharvesting electrical energy, in use, from a potential differencebetween the spaced locations, used for harvesting, which acts as aninput voltage, the harvesting module being arranged for supplying powerto at least one component of the communication apparatus.

The first communication module may be arranged for controlling the loadgenerated by the harvesting module to cause said modulation of theelectric current in the metallic structure at the signalling location.

The harvesting module may be arranged to harvest electrical energy fromdc currents.

According to another aspect of the present invention there is provided adownhole data communication system comprising downhole datacommunication apparatus as defined above located in a well installationhaving metallic structure provided with cathodic protection.

According to another aspect of the present invention there is provided adownhole data communication system for use in a well installation havingmetallic structure provided with a cathodic protection system such thatthere is an electrical circuit comprising the metallic structure and anearth return around which an electrical current flows as a result of thecathodic protection system, the system comprising downhole datacommunication apparatus comprising:

a first communication module located at a first location and comprisingmodulation means for modulating the electrical current at the firstlocation so as to encode data; and

a second communication module located at a second location, spaced fromthe first location, and comprising a detector for detecting the effectof the modulation of the electrical current at the first location so asto extract said data.

The apparatus may comprise a downhole electrical power harvesting moduleelectrically connected between two spaced locations in the wellinstallation and comprising an electrical circuit arranged forharvesting electrical energy, in use, from a potential differencebetween the spaced locations, used for harvesting, which acts as aninput voltage, the harvesting module being arranged for supplying powerto at least one component of the communication apparatus.

The current flow within portions of the metallic structure in regionsbetween the spaced locations, used for harvesting, may be in the samelongitudinal direction.

There may be an uninterrupted current flow path between the spacedlocations, used for harvesting, which is at least partly via themetallic structure.

At least one of the first communication module and the secondcommunication module may be located in an enclosed annulus of the well.

The system or apparatus may comprise a pressure sensor arranged formonitoring the reservoir pressure of the well.

The system or apparatus may comprise a pressure sensor arranged formonitoring the pressure in an annulus of the well.

The system or apparatus may comprise a pressure sensor arranged formonitoring the pressure in an enclosed annulus of the well.

According to another aspect of the present invention there is provided adownhole electrical power harvesting module arranged for electricalconnection between two spaced locations in a well installation andcomprising an electrical circuit arranged for harvesting electricalenergy, in use, from a potential difference between the spaced locationswhich acts as an input voltage.

The harvesting module may be arranged to harvest electrical energy fromdc currents.

The harvesting module may comprise control means for modifying the inputimpedance of the electrical circuit to match the source impedance of theelectrical circuit to optimise power conversion efficiency.

The electrical circuit may comprise a dc-dc convertor.

The dc-dc convertor may be arranged to operate with input voltages abovea minimum threshold, wherein the minimum threshold is not greater than0.5 volt, preferably the minimum threshold is not greater than 0.25volts, and more preferably the minimum threshold is not greater than0.05 volts.

The dc-dc converter may comprise self-start means to allow initiation ofenergy harvesting when the available input voltage is below asemiconductor band gap voltage of components in the dc-dc convertor.

The dc-dc converter may comprise self-start means to allow initiation ofenergy harvesting when the available input voltage is below 0.5 volts.

The dc to dc converter may comprise a step-up transformer.

The self-start means may comprise a Field Effect Transistor arrangedtogether with the step-up transformer to form a resonant step-uposcillator.

The dc-dc convertor may comprise an H bridge of transistors arrangedunder the control of control means for providing an input to the step uptransformer and the self-start means may comprise an auxiliary source ofpower for the control means for allowing start up.

The harvesting module may comprise control means arranged to control theturns ratio of the step-up transformer to modify the load generated bythe dc-dc converter.

A secondary winding of the step-up transformer may comprise a pluralityof tappings and/or the step-up transformer may comprise a plurality ofsecondary windings and the control means may be arranged to selectwindings and/or tappings to provide a desired turns ratio.

The harvesting module may comprise at least a pair of terminals fromwhich connection to the two spaced locations may be made.

The harvesting module may have more than two terminals, wherein each ofthe terminals is for allowing connection to a respective location andthe harvesting module may further comprise switch means for selectivelyelectrically connecting two of the terminals across the electricalcircuit so allowing selection of which of the respective locations theelectrical circuit is connected between.

This allows a set up where multiple contacts to the metallic structuremay be made during installation and after installation a selection ismade as to which contacts should be used. Thus for example the set upmay include one lower connection and two upper connections at differentlocations. Once installed it may be determined that greater power can beharvested if a first of the upper connections is used so this firstconnection may be used. In another case the second upper connection maybe better.

The switch might also be used dynamically in use to switch betweenconnections.

In another case there might be two lower connections as well as orinstead of two upper connections, or there may be other numbers of upperand/or lower connections.

The harvesting module may comprise an energy storage device for storingharvested power. The energy storage device may comprise a charge storagedevice which may comprise at least one capacitor and/or re-chargeablebattery.

The harvesting module may comprise variable impedance means for varyingthe load seen between the two connections.

The variable impedance means may be microprocessor controlled.

The harvesting module may be arranged to use the variable impedancemeans to vary the load so as to optimise energy harvesting.

The harvesting module may be arranged to use the variable impedancemeans to modulate the load so as to communicate data away from theharvesting module.

The harvesting module may comprise a primary battery such that in usepower may be selectively drawn from the power harvested by the circuitand from the primary battery.

According to another aspect of the present invention there is provideddownhole apparatus comprising a harvesting module as defined above and adownhole device to accept power from the harvesting module.

The downhole apparatus may comprise charge storage means and powercontrol means to control power to the downhole device when sufficientenergy is available to power the device.

The downhole apparatus may comprise impedance modulation means forvarying the input impedance of the harvesting module to modulate theload so as to transmit data from at least one of the electrical powerharvesting unit and the downhole device.

The downhole apparatus may comprise modulation means for applying amodulated voltage via the spaced connections so as to transmit data.

The downhole apparatus may comprise a primary battery such that in usepower may be selectively drawn from the harvested power and from theprimary battery.

The downhole device of the downhole apparatus may comprise at least oneof:

a downhole sensor;

a downhole actuator;

an annular sealing device, for example a packer, or a packer element;

a valve;

a downhole communication module, for example a transceiver or repeater.

The valve may comprise at least one of:

a subsurface safety valve;

a bore flow control valve;

a bore to annulus valve;

an annulus to annulus valve;

a bore to pressure compensation chamber valve;

an annulus to pressure compensation chamber valve;

a through packer or packer bypass valve.

According to another aspect of the present invention there is provided adownhole electrical energy harvesting system for harvesting electricalenergy in a well installation having metallic structure carryingelectric current, the system comprising:

a harvesting module as defined above electrically connected to themetallic structure at a first location and to a second location spacedfrom the first location, the first and second locations being chosensuch that, in use, there is a potential difference therebetween due tothe electric current flowing in the structure; and

the harvesting module being arranged to harvest electrical energy fromthe electric current.

According to another aspect of the invention there is provided adownhole power delivery system for powering a downhole device in a wellinstallation having metallic structure carrying electric current, thesystem comprising: a harvesting module as defined above electricallyconnected to the metallic structure at a first location and to a secondlocation spaced from the first location, the first and second locationsbeing chosen such that, in use, there is a potential differencetherebetween due to the electric current flowing in the structure; and

the harvesting module being arranged to harvest electrical power fromthe electric current and supply electrical power to the downhole device.

According to a yet another aspect of the invention there is provided adownhole power delivery system for powering a downhole device in a wellinstallation having metallic structure provided with cathodicprotection, the system comprising:

a harvesting module as defined above electrically connected to themetallic structure at two spaced locations chosen such that, in use,there is a potential difference therebetween due to cathodic protectioncurrents flowing in the structure; and

the harvesting module being arranged to harvest electrical power fromthe cathodic protection currents and supply electrical power to thedownhole device.

According to a further aspect of the invention there is provided amethod of data communication in a well installation having metallicstructure provided with a cathodic protection system such that there isan electrical circuit comprising the metallic structure and an earthreturn around which electrical current flows as a result of the cathodicprotection system, the method comprising the steps of:

modulating the electrical current at a first location to so as to encodedata; and detecting at a second location, spaced from the first, theeffect of the modulation of the electrical current at the first locationso as to extract said data.

One of the locations may be at an out of bore hole location, say, thesurface, another of the locations may be downhole.

The step of modulating the current may, amongst other things, compriseand the modulation means may, amongst other things, be arranged to:

i) where the cathodic protection system is an impressed cathodicprotection system, control a signal source of the impressed cathodicprotection system to directly modulate the cathodic protection signalsapplied to the metallic structure; or

ii) modify the connection between at least one anode and the metallicstructure—thus at least one anode may, for example, be switched into andout of connection with the metallic structure to modulate the electricalsignals or the impedance between the anode and the structure may bevaried; or

iii) alter the impedance of the electrical circuit—this may, forexample, be achieved using a variable impedance means, or by switchingcomponents into and out of connection with the circuit.

Techniques i) and ii) are likely to only be available at an upperlocation, whereas technique iii) is likely to be available downhole andat an upper location.

Communication using this overall idea can be used for one way, say,surface to downhole communication, one way, say, downhole to surfacecommunication and two way communication.

These techniques enable communication as part of a hybrid communicationsystem—i.e. where some parts of the signal channel are provided bymodulating the cathodic protection signals and some by other techniques,such as other wireless techniques including other EM techniques andacoustic techniques.

In each case above the cathodic protection where present may be providedby a passive cathodic protection system where sacrificial anodes areconnected to metallic structure of the well installation or by animpressed cathodic protection system where a protective current isapplied to metallic structure of the well installation.

In the present methods and systems the aim is to make use of existingcathodic protection systems (or other sources of current if available),in particular to make use of existing anodes where present in say subseainstallations and without requiring modification thereto. Thus anodeswhere present will typically be outside, that is above, the bore holeand located in water. Furthermore the anodes will typically be remotefrom the location at which power and/or signalling is required.

Thus any above system may include one or more of: at least one existinganode; at least one anode provided in water, say the body of water inwhich a subsea well installation is provided; at least one anode that isremote from the location at which power and/or signalling is to beachieved using current developed by that anode.

Further any system above may be arranged to enable the transmission ofpower from a location at which current, say CP current, is applied tothe structure to a harvesting and/or signalling location. This beingtrue whether the current is a passive CP current, an impressed CPcurrent, or another applied current. That is to say typically, thesource of the CP current or other current is remote from the harvestingand/or signalling location.

Further, the metallic structure may be uninterrupted in the region ofthe at least one anode and/or the region of the harvesting module.

Where mention is made above of optimisation by modelling for example inrelation to the spacing of connections, use of insulation, choice ofradial only spacing or axial, and the selection of a pre-set harvestingload, at least one of the following parameters maybe used in the model:

1. Attenuation rate at the top of the well derived from casing andtubular dimensions, weights, and material type (resistivity) type andthe resistivity of the overburden (medium surrounding the well).

2. Upper connection location.

3. Lower connection location.

4. Cross Sectional Area and material (resistivity) type of the uppercable used on inputs to the harvester.

5. Number, location, material (electro-potential) and surface area ofthe wellhead anodes.

6. Effective resistance of the well seen from the seabed/wellhead, againderived from casing and tubular dimensions, weights, and material type(resistivity) and resistivity of the overburden (medium surrounding thewell) but this time for the whole completion.

In each case above systems may comprise a primary battery for supplyingpower independently of harvested power. The harvesting module maycomprise the primary battery. Where a primary battery is provided thismay be used preferentially whilst it holds power. It might be used forexample to enable use of a higher date rate at an early stage, thisbeing allowed to fall when only harvested power is available.

According to another aspect of the invention there is provided a wellinstallation comprising metallic structure carrying electric current andany of the above systems or apparatus, thus say at least one of: adownhole electrical energy harvesting apparatus or system; a downholedevice operation apparatus or system; a downhole communication repeaterapparatus or system; a power delivery apparatus or system; or aharvesting module; or a downhole well monitoring apparatus or system; ordownhole communication apparatus or system, as defined above. Such aninstallation may further have a cathodic protection system forprotecting the metallic structure.

Note that in general each of the optional features following each of theaspects of the invention above is equally applicable as an optionalfeature in respect of each of the other aspects of the invention andcould be re-written after each aspect with any necessary changes inwording. Not all such optional features are re-written after each aspectmerely in the interests of brevity.

For example it will be appreciated that any of the systems, methods,apparatus and installations mentioned above may make use of a harvestingmodule having any combination or sub-combination of the features definedabove, and so on.

The well mentioned in any of the above methods, systems, apparatus, orinstallations may be a subsea well.

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying drawings in which:

FIG. 1 schematically shows a well installation including well monitoringapparatus including a downhole power delivery system;

FIG. 2A schematically shows a harvesting module of the power deliverysystem of FIG. 1 and FIG. 2B shows an alternative downhole unit;

FIG. 2C is a schematic circuit diagram of a dc to dc convertor which maybe used in a harvesting module;

FIG. 2D is a schematic circuit diagram of a dc to dc convertor which maybe used in a harvesting module;

FIG. 3 schematically shows a well installation including downholecommunication apparatus which comprises a downhole communicationsrepeater and a downhole power delivery system for powering the downholecommunications repeater;

FIG. 4 schematically shows a well installation including valve operationapparatus comprising a remotely controlled downhole valve and a powerdelivery system for powering the remotely controlled downhole valve;

FIG. 5 schematically shows a well installation including an alternativewell monitoring system comprising a downhole gauge and a downhole powerdelivery system for powering the downhole gauge;

FIG. 6 schematically shows an alternative well installation;

FIG. 7 shows a plot of optimal harvestable power against depth of alower connection for an arrangement of the type shown in FIG. 1;

FIG. 8 shows a flow chart of energy harvesting optimisation;

FIG. 9 shows a flow chart of operation of a downhole unit; and

FIG. 10 schematically shows a well installation including a platform.

FIG. 1 shows a well installation of an oil and/or gas well. As is wellunderstood, such an oil and/or gas well may be a land well or a sub-seawell (meaning a well under any body of water) where the well head isunderwater on the sea, river, lake etc. bed or on a platform. Often wellinstallations are provided with a cathodic protection system. In thecase of land wells this will most likely be in the form of an impressedcurrent cathodic protection system where a protective current is appliedto the metallic structure of the well. On the other hand for a sub-seawell, the cathodic protection will most likely be a passive cathodicprotection system where a plurality of anodes of a relatively reactivemetal, such as a magnesium alloy, are connected to the metallicstructure and exposed to the water in which the well installation issituated

Note that the present techniques are also relevant for water injectionwells—that is wells used to inject water into a reservoir to aidrecovery of oil and/or gas from other wells in the field. Thus a “wellinstallation” in the present specification may be a water injectionwell. Such a well will have a similar construction to the installationsshown in more detail in this application. Similarly the presenttechniques may be used whilst drilling as well as during production andfollowing abandonment. Thus the well installation may be a partiallycomplete installation where drilling is taking place. More generally thepresent techniques may be used during any period of the life cycle of awell installation.

Further, whilst this specific description is written in relation toinstallations where cathodic protection is present and this isparticularly preferred, many of the present systems and techniques alsofunction in other situations where electric current is flowing on themetallic structure and power may be harvested therefrom.

The well installation shown in FIG. 1 comprises a well head 1 anddownhole metallic structure 2 leading down into the borehole of wellfrom the surface S.

The well installation is provided with a cathodic protection system 3A,3B. As alluded to above, this will either be an impressed currentcathodic protection system 3A or a passive cathodic protectioncomprising a plurality of anodes 3B connected to the metallic structureof the well installation, that is to the well head 1 or other metalliccomponents connected thereto.

The downhole metallic structure 2 comprises a first run of metallic pipe21, that is, production tubing, running down into the borehole of thewell. Around this is a first casing 22. Outside this layer is a secondcasing 23 and then a third casing 24. As will be appreciated there is arespective annulus between each run of metallic pipe. Thus there is afirst annulus between the production tubing 21 and the first casing 22commonly referred to as the “A” annulus in the oil and gas industry andindicated by reference numeral A in the drawings.

A second annulus exists between the first casing 22 and the secondcasing 23 commonly known as the “B” annulus and so indicated in thedrawings and a third annulus exists between the second casing 23 and thethird casing 24 commonly known as the “C” annulus and so indicated inthe drawings. Wells also typically can have a further, “D”, annulus, andsometimes even more annuli.

In other situations the metallic structure may comprise other elongatemembers, specifically, one or more of casing, liner, tubing, coiledtubing, sucker rod.

Monitoring apparatus provided in the well installation comprises anelectrical power harvesting module 4 provided, in this embodiment, inthe A annulus. The harvesting module 4 is electrically connected viacables 41 to a pair of spaced locations 41 a, 41 b on the productiontubing 21. In an alternative the harvesting module 4 may be electricallyconnected to one of the locations via a cable but may be electricallyconnected to the other location without a cable. The harvesting module 4may be electrically connected via a conductive housing of (orsurrounding) the harvesting module to one of the locations. Thus onlyone such cable may need to exit the housing.

Note that there is galvanic connection between the harvesting module 4and the metallic structure 21 at the spaced locations 41 a, 41 b.Particularly there is a galvanic connection to the metallic structure21, rather than, for example, an inductive coupling. This simplifies theconstruction and removes engineering difficulties. In the present casethere is a galvanic connection all of the way from the metallicstructure to the inputs of the circuit included in the harvesting modulefor harvesting energy.

Furthermore it will be noted that the metallic structure of the well isgenerally unaffected by the installation of this system. No insulationjoints have been introduced into any of the runs of metallic pipe inorder to make the system effective and the normal flow of cathodicprotection current in the structure has not been altered—other than, ofcourse, the harvesting which is taking place. Thus for example, betweenthe spaced locations the run of metallic structure to which theconnections are made is continuous, more generally all of the runs ofmetallic structure are continuous at these regions. This is notessential for operation, but it is possible and it is the normalprevailing situation in a well installation—ie the standard metallicstructure of the installation has been left unchanged. Similarly thecurrent can and does flow in the same direction in the metallicstructure in the region of the connections and between the connections.Again this is the normal prevailing situation in a well installation,modification to the well installation has been avoided. The current flowmight be in a single run of metallic structure to which the connectionsare made, or jump from one run to another or flow in parallel in severalruns—the point is that an artificial arrangement of metallic structurein the well has not had to be set up to allow the system to work, and assuch there is an uninterrupted current flow path provided by themetallic structure and current flow is in the same longitudinaldirection in the metallic structure.

Note that the “A” annulus is often accessible by cable through the wellhead 1. However, it is still advantageous to use the presentarrangements as they minimise the number of penetrators in the wellhead, reducing risk and expense and/or freeing up a penetrator for otheruse.

The monitoring apparatus further comprises a downhole gauge 5 which isprovided deeper in the well than the harvesting module 4 and isconnected thereto via a cable 42. In this embodiment the downhole gauge5 is provided just above a packer P. Typically the cables 41 connectingthe harvesting module 4 to the production unit 21 will be tubingenclosed conductors (TEC) as typically used in the oil and gas industryand the cable 42 connecting the harvesting module 4 to the downholegauge 5 will also be a tubing enclosed conductor (TEC). Moreovertypically the cross-sectional area of the conductor in the lengths ofcable 41 connecting the harvesting module 4 to the production tubing 21will have a larger cross-sectional area than that of the cable 42connecting the harvesting module 4 to the downhole gauge 5.

Where cathodic protection is provided in a well installation, thepotential of the metallic structure of the well is taken to asufficiently negative potential at the point of injection, say the wellhead 1, such as to suppress corrosion at the well head and at otherpoints along the downhole metallic structure 2 as it descends into thewell. However the magnitude of this negative potential will decrease asone progresses further down into the well due to the losses in thesystem. Therefore the potential of the metallic structure 2 near thewell head will be more negative than at deeper locations in the well.Thus when cathodic protection currents are flowing in the wellinstallation there will be a potential difference between the location41 a where the first of the cables 41 from the harvesting module isconnected to the production tubing 21 and the location 41 b where theother of the cables 41 from the harvesting module 4 is connected to theproduction tubing 21. Thus the harvesting module 4 will see a potentialdifference across it and as such can extract energy from the cathodicprotection currents.

It will be noted that extracting energy will use power from the cathodicprotection system however the impact on the effectiveness of thecathodic protection system or any acceleration of the corrosion of theanodes will be negligible. Typically cathodic protection currents willbe of the order of 10 Amps whereas the present systems might extract say10-100 milli Amps. Thus the amount of current extracted is well withinthe tolerance usually allowed for when developing cathodic protectionsystems. If desired an increased level of impressed current can beprovided or the number of anodes provided could be increased beyond thenorm. This would increase the cathodic protection current and henceimprove harvesting.

Electrical power may be harvested from the system at the downholelocation of the harvesting module 4 and this harvested power may be usedfor other purposes.

In the arrangement of FIG. 1 this harvested power is used to power thedownhole gauge 5 and allow extraction of readings therefrom andcommunication of those readings to the surface S.

In the present embodiment an upper communication unit 6 is provided forcommunicating with the harvesting module 4 and downhole gauge 5. In thisinstance the upper communication unit 6 is provided at the surface S—inthis case the land surface.

It will be appreciated that arrangements such as the present one may beused in place of a conventionally installed permanent downhole gauge(PDG) with the advantage that use of a penetrator through the well headcan be avoided, whilst life of well monitoring will be feasible in manycases. Monitoring may be of reservoir pressure where desired orsimilarly of the pressure in an enclosed annulus to, for example, helpdetect any leak, issue, or failure in the system. The sensor andharvesting module may be located in the enclosed annulus, in such acase.

All of these options are possible in say a subsea well installationwhere there will normally be a ready source of current to beharvested—ie CP current, typically generated by sacrificial anodeslocated in the water in which the subsea installation is provided, andwhere other power and signalling options are more problematic.

In a well with a subsea wellhead, conventionally it is not generallypossible (practically/cost effectively) to provide hydraulic orelectrical connectivity with the outer annuli (B, C etc). Particularlywhere these annuli are sealed at their base it is useful to monitor andoptionally control pressure in these annuli, for instance, to reduce therisk of high pressures causing collapse of the casing.

In particular the flow, or drilling of the well may increase thetemperature of the sealed outer annulus and hence increase the pressuretherein. The ability to monitor pressure in such a case and optionallycontrol pressure in such a case (such as with a vent valve betweenannuli, as mentioned elsewhere) is beneficial. In particular, monitoringthe pressure in an enclosed annulus may permit production at higherrates than those achievable if modelling of the expected pressure risealone is used as use of modelled pressure would require greater safetymargins and potentially correspondingly reduced production rates. Aswill be appreciated the present techniques can facilitate suchmonitoring and/or control.

Another particular implementation of the present techniques will includea sensor module located in the same location as is most usual for aconventional permanent downhole gauge and provided for the same purposeas is most usual for a conventional permanent downhole gauge.

Thus the sensor module may be disposed in the A annulus and arranged formonitoring the reservoir pressure by sensing the pressure in the tubingvia a pressure communication port through the tubing so allowinginference of the reservoir pressure based on the sensed pressure andtaking into account static pressure and flow effects. As is the casewith a conventionally used PDG, reservoir pressure will generally beinferred in this way rather than directly measured—positioning a sensordirectly in the reservoir is generally not feasible—as will also beappreciated “monitoring reservoir pressure” covers use of suchmeasurement techniques.

A harvesting module may also be provided at the location of the sensormodule.

Different techniques may be used for allowing the extraction of datafrom the downhole gauge 5 towards the surface.

In the present embodiment the harvesting module 4 is arranged to accepta signal from the downhole gauge which is indicative of the parameter tobe measured, for example, pressure and/or temperature and to transmitthis data towards the surface by virtue of modulating the load which theharvesting module 4 creates between the spaced connections 41 a and 41b. In turn this change in load will change the amount of current drawnfrom the cathodic protection currents applied to the system. This inturn is detectable at the surface or other convenient location by thevirtue of a change in the potential of the metallic structure at thesurface or the other convenient location. It may be detected bydetecting for example, the change in potential at the well head 1 or bydetecting the voltage across, or a current seen by, a power supply usedin an impressed cathodic protection system 3A. In the present embodimentthe effect of the modulation is detected by the upper communicationsunit 6, monitoring the potential of the well head relative to areference earth, to extract the pressure and/or temperature measurementdata.

Preferably the spacing between the spaced connections 41 a, 41 b is atleast 100 metres and more likely in the region of 300 to 500 metres. Theoptimal spacing for the spaced connections 41 a, 41 b may be determinedby modelling for a given installation. As the distance between theseconnections is increased this tends to increase potential differencebetween the connections (although the rate of increase of potentialdifference decreases as the depth of the lower connection is increased).On the other hand, as the spacing increases the total length and henceresistance of the cables 41 increases. Thus in most systems there willbe an optimal spacing.

FIG. 2A shows the harvesting module 4 of the apparatus shown in FIG. 1in more detail. In this embodiment the harvesting module 4 has a pair ofterminals 43 a, 43 b to which the respective cables 41 are connected.There is galvanic connection between the metallic structure and theterminals 43 a, 43 b. Connected between these terminals 43 a, 43 b is alow voltage dc to dc converter for harvesting the electrical energywhere potential difference is seen across the terminals 43 a, 43 b. Thedc to dc converter 44 is connected to a charge storage means 45including at least one low leakage capacitor and connected to andcontrolled by a microprocessor driven central unit 46. The chargestorage means 45 and central unit 46 are also connected via a respectiveterminal 43 c to the length of cable 42 which leads to the downholegauge 5. In an alternative the charge storage means 45 might bedispensed with—ie: enough power might be harvested to allow continuousoperation as and when required.

In operation, the central unit 46 controls the operation of the dc to dcconverter 44 so as to optimise the load which it presents to the currentseen by the harvesting module 4 due to the cathodic protection currentsin order to maximise the energy which may be harvested and used orstored in the charge storage means 45. Note that the central unit may bearranged to selectively use and/or deliver harvested energy directlywhen appropriate, and store energy and extract stored energy whenappropriate.

Note that in an alternative the microprocessor driven central unit 46may be replaced by alternative electronics including say an analoguefeedback circuit, or a state machine or even a fixed harvesting loadbased on modelling for the particular installation.

When stored energy is to be used, power from the charge storage means 45is fed via the cable 42 to the downhole gauge 5 and readings from thedownhole gauge 5 are acquired by the central unit 46 via the cable 42.The central unit 46 also controls operation of the dc to dc converter 44to modulate the load which is introduced between the terminals 43 a and43 b in order to send signals back to the surface carrying readings fromthe downhole gauge 5 as described above.

Note, that in the present embodiment the dc to dc converter 44 andcentral unit 46 together act as a variable impedance means by virtue ofthe central unit 46 controlling the operation of the dc to dc converter44 to introduce variable impedance between the terminals 43 a and 43 b.

Note that in alternatives, rather than a sensor being provided in aseparate downhole gauge 5, an appropriate sensor may be provided at thesame location as the harvesting module 4.

In particular, a downhole unit 4 a as shown in FIG. 2B may be providedwhich comprises both a harvesting module 4 and at least one downholedevice to be powered. In this case the downhole unit 4 a includes apressure sensor 47 and a communications unit 48.

In such case there may be no secondary cable 42 leading away from thedownhole unit 4 a. On the other hand in some other cases the downholeunit 4 a might still be used to power an external device even ifincluding its own sensor 47 and/or communications unit 48 and thus theremight be a secondary cable 42.

In alternatives, rather than communicating to the surface using the loadmodulation technique as discussed above, the downhole unit 4 a might useits own communications unit 48 for communicating back towards thesurface. Such communication might be in the form of the EM communicationsignals which may be applied back to the downhole metallic structure 21via the cables 41. In other cases the communications unit 48 provided inthe downhole unit 4 a might be an acoustic communications unit forapplying acoustic signals to the metallic structure 21 for transmissionback towards the surface. In such a case then an upper communicationsunit would be arranged for receiving acoustic signals. It will beappreciated that two way communication may be provided as and whendesired over any or all parts of the communications channels. Furthertwo communication techniques may be used parallel in any leg of thecommunications channels—thus EM signals and acoustic signal might beused side by side.

In further alternatives the harvesting module 4 or downhole unit 4 a maycomprise at least one power converter for controlling the voltage atwhich the power is harvested for delivery to the charge storage means 45and/or other components such as the central unit 46. It may be desirableto store energy at a different voltage than that at which it isharvested and/or different from that at which it is used by the centralunit 46 or other components. For example, it may be desirable to storethe power at a higher voltage than that at which it is harvested and/orconsumed. This can be useful, for example, if there is a large draw onthe stored power during for example transmission.

A possible implementation for a dc to dc convertor is to use acommercially available integrated circuit. An alternative is to producea similar circuit using discrete components. To provide effectiveperformance a dc to dc convertor that can cope with low input voltagesis desirable. One way to achieve this is to use a Field EffectTransistor, such as JFET switch, to form a resonant step-up oscillatorusing a step-up transformer and a coupling capacitor. In order to helpoptimize energy harvesting the turns ratio on the transformer may beselected, preferably dynamically selected during operation. A pluralityof tappings may be provided on the secondary of the transformer whichmay be selectively used to provide respective turns ratios.

A processor, such as that of the central unit may be arranged to controla switch to dynamically select the respective tappings and hence controlthe load generated by the dc-dc convertor.

FIG. 2C shows a schematic circuit diagram for a possible implementationof a resonant step-up oscillator of the type described above. Theavailable input potential difference may be connected across the inputterminals as Vin and the output Vout is seen across the outputterminals. The circuit comprises a Field Effect Transistor 201, a stepup transformer 202 which together act as an oscillator and a rectifyingoutput arrangement 203 comprising a crossed diode pair 206 andrespective coupling capacitors 205. A primary winding 202 a of thetransformer 202 is connected in series with the FET 201 and the inputVin is applied across these. The gate of FET 201 is connected to thesecondary winding 202 b of the transformer 202. The output Vout is seenacross the coupling capacitors 205 which are each connected across thesecondary winding 202 b via the respective diodes 204.

The secondary winding 202 b of the transformer 202 comprises a pluralityof tappings 202 c which can be selected using switch 206 so allowingadjustment of the turns ratio. The switch 206 can be controlled by amicroprocessor, in this case the central unit 4 b.

This type of dc to dc convertor arrangement is able to function evenwhen the potential difference seen across the terminals (input voltage)is low, that is 0.5V or below. In practical examples the input voltagemay be less than 0.25V and perhaps even less than 0.05V. As this is verylow compared with semiconductor band gap voltages (say 0.7V) many typesof dc to dc convertors will not function to allow energy harvesting atsuch input voltages. However, dc to dc convertors based on the aboveprinciples can function at even such low voltages. Such a dc to dcconvertor can be considered to include start up means arranged to allowoperation when the input voltage is 0.5V or below as well as at highervoltages.

An alternative approach is to provide a circuit with a separate powersource to act as part of a start up means. Thus, for example, a primarybattery may be provided to start up the system after installation.Furthermore stored energy in an energy store might be used to restartthe system if energy harvesting temporarily stops.

FIG. 2D shows a schematic circuit diagram for a possible implementationof a dc to dc convertor operating on such a basis. The dc to dcconvertor of FIG. 2D comprises an H bridge 207 of transistors 207 aacross which the input voltage is connected. The gates of thetransistors 207 a are connected to a control unit 208 which is arrangedto control the switching of the transistors 207 a to generate an acoutput. The ac output of the H bridge 207 is connected across a primarywinding 202 a of a step up transformer 202.The secondary winding 202 bof the transformer 202 is connected to a rectifier 209. One output ofthe rectifier 209 is connected via a diode 204 to the input of a powersupply unit 210 and the other output is connected to ground. Alsoconnected to the input of the power supply unit 210 via another diode204 is a battery 211.

The power supply unit 210 is arranged to power the control unit 208. Inorder to start up operation the power supply unit 210 may use power fromthe battery 211. Once energy is being harvested by the dc to dcconvertor then the power supply unit 210 may use power received from therectifier 209—ie harvested power.

Whilst in the present embodiment power is used directly as harvested, inalternatives harvested energy may also be stored in a storage means andused from the storage means. As described elsewhere in this application,the storage means may, for example, include at least one low leakagecapacitor and/or at least one rechargeable cell. Where energy is storedthis allows a mechanism to restart the system if harvesting is ceased atany point after the battery 211 has discharged.

The battery 211 may be a primary (one shot) battery, or may be are-chargeable battery provided it is charged at the time ofinstallation. Where the battery is a re-chargeable battery, in someimplementations the power supply unit 210 may be arranged to storeenergy in it when available, alternatively it may be more convenient toprovide a separate energy storage means (which might include arechargeable battery).

Note also that in a further alternative a dc to dc convertor of the typeshown in FIG. 2D may be arranged to allow control of the load generatedby the dc to dc convertor. Thus for example, a similar arrangement tothat shown in FIG. 2C may be used where the secondary winding 202 b hasmultiple tappings and a switch is provided to allow selection of thetappings. This switch could sit between the windings and the input tothe rectifier 209. In another alternative separate secondary windingscould be provided rather than multiple tappings, to achieve a similarresult. The switch can be controlled by a control unit as in the case ofthe arrangement of FIG. 2C.

Note also that in other embodiments the harvesting module 4 and downholegauge 5 (or downhole unit 4 a) may be provided in other annuli withinthe well installation rather than the A annulus. Further the gauge maybe arranged to sense a parameter in a different annulus than the one inwhich it is located.

For example, these components may be provided in the B or C annulus anda gauge located in say the B annulus may be arranged to sense one ormore parameter in the A annulus, the B annulus, the C annulus or anycombination thereof. It is noted that these are locations where it isgenerally not possible, or at least undesirable, to try to providedirect cable connections from the surface. Thus the present techniquesgive rise to the possibility of monitoring say pressure in the B or Cannulus for the life of a well installation where this would bedifficult and/or expensive using conventional power delivery methods.The present techniques avoid the use of penetrators through the wellhead which can reduce risk and cost. They also provide relativelysimple, neat and easy to install solutions.

FIG. 3 shows a well installation similar to that of FIG. 1 but includinga downhole communications repeater 7 rather than a downhole gauge. Therepeater 7 is provided in the B annulus along with a harvesting module 4of the same type described above in relation to FIGS. 1, 2A to 2D. Hereagain the harvesting module 4 harvests power from the cathodicprotection currents in the metallic structure 2 and provides this powerto the downhole communications repeater 7.

The structure and operation of the well installation, cathodicprotection system and power delivery system in the arrangement of FIG. 3is substantially the same as that in the system described with referenceto FIGS. 1, 2A to 2D.

The only difference resides in the fact that the downhole componentdelivered power by the power delivery system is a communicationsrepeater 7 rather than the downhole gauge 5.

Thus, detailed description of the well installation and power deliverysystem is omitted here in the interests of brevity. Where components arereferred to in respect of this embodiment which are the same as that inFIGS. 1 and 2A to 2D, the same reference numerals are used.

The downhole communications repeater 7 is arranged to pick up signalsfrom the downhole metallic structure 2 in the region of the repeater 7and transmit the relevant data onwards towards the surface. In thisembodiment the signals are applied to the downhole metallic structure 2as EM signals by a transmission tool 71 located further down in thewell, for example in the production tubing 21. Correspondingly therepeater 7 is arranged to pick up EM signals.

In alternatives a different type of transmission tool may be providedfor sending signals which are picked up by the repeater. Such a toolmay, for example, be disposed outside of the tubing.

In alternatives the communications repeater 7 may be arranged to pickacoustic signals from the downhole metallic structure 2 which have beenapplied further downhole.

Similarly, the downhole communications repeater 7 may be arranged toapply acoustic signals to the downhole structure 2 for transmissiontowards the surface or arranged to apply EM signals to the downholemetallic structure 2 for transmission to the surface or to make use ofthe impedance modulation signalling technique described above.

Thus, for example the communications repeater 7 may pick up signals atits location and transmit these along the cable 42 to the harvestingmodule 4 by applying signals thereto or by modulating the load which itputs on the power supply in the harvesting module 4. Similarly, theharvesting module 4 may be arranged to apply signals to metallicstructure 2 for transmission towards the surface or be arranged tomodulate the load which it generates between the spaced connections 41a, 41 b for detection at the surface by the upper communication unit 6.

Note that in the case of the provision of a downhole communicationsrepeater 7, EM signals may, for example, be picked up and/or applied bythe repeater 7 using spaced contacts made to the metallic structure, orusing an inductive coupling comprising a toroid or signalling across aninsulation joint should one be available and so on. Similarlyconventional acoustic signal pick up and application techniques may beused.

In alternatives there may be communication from the surface downwards todownhole locations and in general two way communication. Thus therepeater 7 may act as a repeater in both directions. Again twocommunication techniques may be used in parallel on at least one leg ofthe channel to provide redundancy.

Note also that the downhole communications repeater 7 may be provided ina location such as not to be in the product flow whilst allowing life ofwell operation.

Two specific examples relating to FIG. 3 are:

1. The repeater 7 comprises a continuously powered EM receiver at 3-500m depth which either receives and decodes messages or simplycontinuously re-transmits using load impedance modulation at a higherfrequency, raw data/signal for decode at the surface.

2. The repeater 7 comprises a continuously powered acoustic receiver at3-500 m depth which receives and decodes messages and then re-transmitsdata to surface using load impedance modulation.

Note that in both these cases the repeater 7 maybe provided in adownhole unit with the harvesting module, or be separate therefrom.Again the repeater may be a two way repeater.

In any of the systems described in this specification the devices may bearranged to manage the power budget, i.e. use less energy overall, byusing intermittent operation of the components such as EM or acousticreceivers and/or transmitters.

FIG. 4 schematically shows a well installation including a remotelycontrolled valve and a power delivery system of the same general type asdescribed above.

The general structure and operation of the well installation and thepower delivery system is again substantially the same as that describedabove in relation to the arrangements shown in FIGS. 1, 2A to 2D. Thusdetailed description of those common elements is omitted here for thesake of brevity and the same reference numerals are used to indicatethose features which are in common between the two embodiments.

In this embodiment the well installation comprises a first hydraulicallyoperated sub-surface safety valve SSSV provided in the production tubing21 as is conventional.

However, here an additional subsurface safety valve 8 is provided alsowithin the production tubing 21, but further down in the well. Thus inthe present case the second subsurface safety valve 8 is provided as anadditional safety or fallback measure. However, in alternatives it mightbe that the hydraulically operated subsurface safety valve SSSV could bedispensed with.

The second subsurface safety valve 8 is powered and operated by makinguse of a power delivery system. In particular a harvesting module 4 isconnected to the second sub-surface safety valve 8 via a cable 42 andthe harvesting module is arranged to issue power and control signals tothe second subsurface safety valve 8 via the cable 42. Thus energy isharvested from the cathodic protection currents running in the downholestructure 2 and this is used to both control and operate the secondsubsurface safety valve 8.

Such a subsurface safety valve 8 may be located deeper into the wellthan a traditional hydraulically operated subsurface safety valve SSSV.This is because it is not subject to the same range limits ashydraulically driven systems—there is no requirement to drive hydraulicfluid to it. It will be noted that here control signals for the secondsubsurface safety valve 8 may be transmitted by the upper communicationsunit 6 via the metallic structure of the well 1, 2 for detection by theharvesting module 4 and onwards transmission to the subsurface safetyvalve 8. In some circumstances the valve 8 may be caused to operate in afail safe mode such that the valve will close in the absence of powerand/or control signals. Note of course that in an alternative the valve8 and harvesting module might be provided as part of a common downholetool 4 a. Further in some cases power for closing the valve may comefrom another source, with the downhole power delivery system supplyingpower for controlling operation and/or operating a trigger mechanism.

FIG. 5 shows an alternative well installation including well monitoringapparatus. Here again there are similarities with the arrangement shownin and described with reference to FIGS. 1, 2A to 2D. Again there is aharvesting module 4 provided within the downhole metallic structure 2and connected to spaced locations on the downhole structure 2 andmoreover there is a downhole gauge 5 connected to the harvesting module4. In this instance the harvesting module 4 and downhole gauge 5 areboth provided in the B annulus to provide monitoring of conditions inthis annulus. The downhole gauge 5 may, for example, comprise a pressureand/or temperature sensor.

In this instance the spaced locations 41 a, 41 b are provided ondifferent runs of the downhole metallic structure 2. In particular inthis embodiment, a first of the connections 41 a is made to the secondcasing 23 whilst the other of the connections 41 b is made to the firstcasing 22. The system works on a similar principle as discussed aboveand therefore relies on a potential difference existing between thesetwo connections 41 a, 41 b. In the present embodiment this potentialdifference is realised by virtue of insulating the two runs of metallicstructure 22, 23 from one another in at least the region of theseconnections. This means that there is a different passage to earth forthe cathodic protection currents from the two runs of metallic structure22, 23. In the present embodiment the means of insulating the two runsof metallic structure 22, 23 from one another comprise an insulatingcoating 91 provided on the outer surface of the first casing 22 and aplurality of insulating centralisers 92 provided on the first casing 22to keep this separated from the second casing 23.

Preferably this insulation 91 and these centralisers 92 will be providedover a length of the first casing 22 of at least 100 metres and morelikely 300 to 500 metres. Where desirable and practical, insulatingspacers may be mounted on the outer run of metallic structure formingthe annulus. Thus for example, mounted on the second casing 23 in theabove example. Note that the insulation need not be entirely continuousto provide a useful effect. The creation of a different path to earth isthe aim. Thus whilst, say the insulation may be provided over 100 m, itmay not be continuous, or provide continuous insulation over thisdistance.

The benefit of the arrangement shown in FIG. 5 is that the long lengthsof cable 41 between the harvesting module 4 and the metallic structure 2required in the arrangement shown in FIG. 1 can be dispensed with. Thismeans that the system may be easier to install. For example the systemmay be deployed by virtue of a housing for the harvesting module 4 beingmounted on a piece of metallic pipe and provided with a sliding contactfor contacting another piece of pipe across the annulus. To furthersimplify the position the downhole gauge 5 may be dispensed with and asensor provided along with the harvesting module 4 in a downhole unit 4a. Such an arrangement can reduce rig time required for installation.

Thus in some circumstances the provision of the insulation means 91, 92may be preferable to the provision of the cables 41. Which system ispreferable for a given installation may be determined by externalfactors concerning the installation or perhaps by modelling theparticular installation.

In a typical case however, the arrangement of FIG. 1 is likely to givebetter performance than that of FIG. 5, where it is feasible to use thatsystem.

In an arrangement of the type shown in FIG. 5 relatively higher currentbut relatively lower potential difference is likely to be seen by theharvesting module. Thus in a FIG. 5 arrangement the potential differencemight be say 10-20 mV and current say 1 Amp. On the other hand in a FIG.1 arrangement, the potential difference might be say 100-200 mV and thecurrent say 100-150 mAmps. Higher potential difference is achieved bythe greater spacing given by the cable(s) 41 in the FIG. 1 arrangement,but the lower current is caused by the resistance of the cable(s).

Other than this difference in how the connections are made and apotential difference is achieved, and the different attending benefitsand disadvantages, the structure and operation of the system as shown inFIG. 5 is similar to that as shown in FIG. 1. Accordingly the differentalternatives which are explained above in relation to FIGS. 1 to 4 arealso applicable where a system such as that shown in FIG. 5 is used.

That is to say an insulation and connection arrangement as shown in FIG.5 may be used in each of the implementations shown in FIGS. 1, 3 and 4and similarly the different forms of harvesting module 4 and, downholeunit 4 a discussed above may be used in an arrangement such as thatshown in FIG. 5.

Note that in some circumstances it may be desirable to use the presentpower delivery systems to provide a wireless ready well installationeven if there is no intention to use the wireless capabilities when thewell is first installed.

Thus the arrangement shown in FIG. 3 where a communications repeater 7and associated power delivery system is included in the B annulus may beprovided when a well is first installed to make the well wireless ready.This will facilitate communication to the surface if at a later time itis decided to use, for example, a downhole wireless signalling tool 71to signal to the surface. Note here again we are referring to“wirelessness” between downhole and the exterior—i.e. withoutcables/wires going through the well head.

In other circumstances the present systems may be retro-fitted. Forexample, a system such as that shown in FIG. 1 installed in the Aannulus may be retro-fitted when production tubing is replaced. Inanother case a system could be installed in the main bore of theproduction tubing. Note that importantly each of the arrangements andtechniques described in the present specification avoid the need for acable to penetrate through the well head 1. Thus these systems can beused where no penetrator is available or the use of one is unattractive.

Whilst the arrangement in FIG. 4 shows the provision of an additionalsubsurface safety valve 8, in other circumstances a different type of(possibly remotely operated) valve or component may be provided. Forexample an arrangement of the type shown in FIG. 4 may be used with anannulus vent valve provided in a well to allow controlled fluidcommunication or venting between one annulus and another or between anannulus and the bore. The valve could comprise a gas lift injectionvalve for allowing gas into the bore of production tubing from the Aannulus. Similarly the valve may be a packer, a through packer valve ora packer by-pass valve. Again for allowing venting of a particularannulus under control from the surface. In another example the valve maycomprise a flow control valve to either control contribution from a zoneor provide a means to enable improved pressure build up data capture byremoving the effect of well bore storage. Note that the valve in eachcase may be flow control device which may not allow complete shuttingoff of flow but say act as a variable choke.

The valve or component in each case may be a wirelessly controlled valveor component.

In another alternative the present techniques may be used forcommunication with and/or control of a tool supported by awireline/slick line or attached to coiled tubing in the productiontubing 21. That is to say, such a tool may be arranged to apply signalsto and/or pick up signals from the tubing which signals pass through therepeater 7.

With systems of the present type one might be able to extract power atthe level of perhaps 50 mW. Thus the amount of power which may beextracted is not particularly large, but what is of interest is the factthat this power can be available throughout the life the well and issufficient for performing useful functions such as controlling downholedevices, taking important measurements and allowing transmission ofthese measurements to the surface.

Note that in general in embodiments of the general type shown in FIGS. 1to 4 harvesting efficiency will be dominated by the cross-sectional areaof the cable(s) 41 and the source impedance provided by the connections41 a and 41 b is low. This means that if multiple harvesting systems areincluded in one well installation there is little reduction inperformance of any one harvesting module 4. Note that in general anyadditional harvesting system would have its own cables 41 whereappropriate. This is on the basis that losses in cable mean thattypically little would be gained by having more than one harvestingsystem sharing a cable.

In general a plurality of harvesting modules of any of the typesdescribed above may be provided in one well installation. Thus, forexample, a gauge may be provided to monitor conditions in the productiontubing, a gauge may be provided to monitor an annulus, and a valve maybe provided, all of which have power supplied from a separate respectiveharvesting module. Similarly any one harvesting module may be used topower a plurality of devices. In some instances each device may havededicated cable from the harvesting module. In other instances there maybe a multi-drop system where one cable from the harvesting module isused to connect to a plurality of downhole devices. The multi-dropsystem may be arranged to allow power delivery and communications withthe plurality of downhole devices. As such, the cable may carry powersignals, communication data and addressing data. Correspondingly theharvesting module may be arranged to administer the multi-drop system.

Note that whilst in the embodiments above the cables 41, 42 run withinunobstructed annuli, in other cases one or more of the cables 41, 42 maypass through a packer (including a swell packer), cement or otherannular sealing device.

It will also be appreciated that in at least some cases features of thepresent systems and apparatus may have distributed form. Thus say, forexample, the harvesting module may be provided in a plurality ofseparate parts, components, or sub-modules that may be differentlylocated.

FIG. 6 shows an alternative well installation which has similarity withthe installation shown in FIG. 1 and the same reference numerals areused to indicate the features in common with the embodiment of FIG. 1and detailed description of these common features is omitted.

The well installation shown in FIG. 6 helps to illustrate in more detailsome of the alternatives described above in relation to each of the wellinstallations shown in and described with reference to FIGS. 1 to 5.

The well installation includes monitoring apparatus in the same way asFIG. 1. Thus there is a harvesting module 4 connected via cables 41 to apair of spaced locations 41 a and 41 b. However, in this case a first ofthe locations 41 a is on the production tubing 21 and thus a first ofthe cables 41 is connected to the production tubing whilst the second ofthe spaced locations 41 b is on the casing 22. Thus there is both axialand radial spacing between the connections 41 a, 41 b in this embodimentand thus the harvesting module 4 is connected across the “A” annulus.Furthermore, insulation 91 is provided on the production tubing 21 inthe region of the second connection 41 b and extends axially either sideof this. Note that in another alternative, one connection might be tothe formation rather than to the metallic structure. In some cases allof the apparatus of the power delivery system could be provided outsideof the casing—i.e. between the casing and formation. This will generallybe undesirable from a risk/difficulty in installation point of view, butis a possibility.

Further, in the present embodiment there are second and third harvestingmodules 4′ and 4″ (which are part of respective downhole units) providedin the “A” annulus. In this embodiment each of these other harvestingmodules 4′, 4″ makes use of the same first cable 41 and as such oneterminal of each of the harvesting modules 4′, 4″ is connected to thefirst connection point 41 a. Note that in other embodiments separatecables could be used for making these connections to the firstconnection point and this would be preferable leading to improvedperformance. A single upper cable, as shown, whilst possible is unlikelyto be used, but helps simplify the drawing. In some cases a plurality ofharvesting modules may be provided which are distributed acrossdifferent annuli.

In the present embodiment the first harvesting module 4 is connected viaa secondary cable 42 to a downhole gauge 5 similarly to the embodimentsshown in FIG. 1. However, here the downhole gauge 5 is located below apacker P and the cable 42 passes therethrough. The gauge 5 in this caseis arranged for taking pressure and/or temperature measurements ofconditions inside the production tubing 21 through a port 21 a providedin the wall of the production tubing 21. That is to say although thedownhole gauge 5 is provided in the “A” annulus it is arranged formeasuring parameters within the production tubing 21.

Further, in this embodiment second and third downhole gauges 5′ and 5″are provided. In this embodiment each of the downhole gauges 5, 5′, 5″is connected to the harvesting module 4 via the same secondary cable 42.Thus this is a multi-drop system and the cable 42 is used for carryingpower signals, control signals, parameter data and addressing data toallow powering of each of the gauges 5, 5′, 5″ as well as extractingreadings therefrom.

Note that in alternative embodiments a number of downhole gauges orother downhole devices may be powered from one harvesting module 4 viaindividual dedicated cables 42 rather than a single cable as in thepresent embodiment. Further, as alluded to above, whilst in the presentembodiment there are a plurality of gauges which are run off oneharvesting module, in other embodiments one harvesting module may beused for powering different types of downhole device. Thus oneharvesting module, for example, might be used to power a downhole gauge,a downhole repeater and a downhole valve.

In the present embodiment the second harvesting module 4′ is part of adownhole tool which comprises both a harvesting module and a sensor. Inthe present case the sensor is arranged for measuring parameters in the“B” annulus via a port 22 a provided in the first casing 22. Thus, forexample, the sensor in the second harvesting module 4′ may be arrangedfrom measuring pressure and/or temperature in the “B” annulus.

Furthermore, in the present embodiment the third harvesting module 4″ isagain part of a downhole tool comprising, in this case, the harvestingmodule and a communication unit for communicating with sensors 605provided in the “B” annulus and the “C” annulus. Here, communicationbetween the sensors 605 and the second harvesting module 4″ is viawireless means. Thus, for example, there may be inductive signalling oracoustic signalling between the sensors 605 and the harvesting module4″. The sensors 605 may be placed physically as close as possible to theharvesting module 4″.

It will be appreciated that once data is at the upper communicationsunit 6, it may be transmitted onwards to any desired location usingstandard communication techniques such as mobile communicationtechniques, the internet and so on to a desk location D for furtherprocessing and/or review. Of course wired connections might also beprovided between the desk location and the upper communication unit 6.

Furthermore, data may also be sent from the desk location D to the uppercommunication unit 6 for transmission downhole. Thus, for example,control signals may be transmitted from a desk location D via the uppercommunications unit 6 downhole to control operation of a harvestingmodule or sensor or downhole valve or repeater or so on and similarlyany desired data may be sent in this fashion downhole.

In a further alternative, insulation may be provided on the outside ofthe outermost casing, for example, the third casing 24 in the embodimentshown in FIG. 6 in the region near the well head 1. This can help drivethe maximum negative potential caused by the cathodic protectioncurrents further down into the well. This is by virtue of minimising theleakage in this region near the well head. Thus providing insulation onthe outermost casing can help allow the uppermost connection 41 a to bepositioned lower in the well without significantly reducing theeffectiveness of the system. If one considers the potential decay curve,then by providing insulation on the outermost casing 24, the negativepotential will decay very slowly in the insulated region near the wellhead and then begin to decay more quickly once the uninsulated regionhas been reached.

FIG. 7 is a plot showing an example of how the optimal power availablefor harvesting in a well installation varies with depth in the well. Asmentioned above, due to the increase in potential difference which isavailable as the spacing between the connection increases on the onehand and the resistance of the cable on the other hand, there tends tobe an optimum depth for the lower connection 41 b, or to put thisanother way an optimum spacing between the two connections 41 a and 41b. The plot shown in FIG. 7 relates to a position where the upperconnection 41 a is approximately 5 metres below the well head and thusin the region of the liner hanger. In this example it can be seen thatthe optimum depth of the lower connection is in the order of 550 metresdown in the well. However, it can also be seen that a significantproportion of the optimum power can be obtained at depths between say300 and 950 metres. In general terms it would be desirable to minimisethe length of the cable whilst achieving an optimum power harvestingsuggesting minimising the depth of the second connection. However theremay be some circumstances where advantage of the fact that theharvesting module may be placed deeper in the well can be taken.

The optimal location for the upper connection may depend on the wherethe CP current (or other current) is injected and where the current is amaximum, or the potential caused by the current is a maximum. Thepresent methods and systems may include steps of first determining wherethe applied current (or potential) has maximum magnitude and choosingthe location for the upper connection in dependence on this.

Where the well is a land well the upper connection may be within 100 mof the surface, preferably within 50 m.

Where the well is a subsea well the upper connection may be within 100 mof the mudline, preferably within 50 m.

As mentioned above, whilst the above description refers to harvestingfrom cathodic protection currents and this is preferred, if othercurrents are present in the metallic structure, they may be equallyused.

It will be appreciated that whilst particular examples are given above,in general any of the components of the system may be provided in anyavailable annuli.

Where mention is made above of optimisation by modelling for example inrelation to the spacing of connections, use of insulation, choice ofradial only spacing or axial, and the selection of a pre-set harvestingload, at least one of the following parameters maybe used in the model:

-   -   1. Attenuation rate at the top of the well derived from casing        and tubular dimensions, weights, and material type (resistivity)        type and the resistivity of the overburden (medium surrounding        the well).    -   2. Upper connection location.    -   3. Lower connection location.    -   4. Cross Sectional Area and material (resistivity) type of the        upper cable used on inputs to the harvester.    -   5. Number, location, material (electro-potential) and surface        area of the wellhead anodes.    -   6. Effective resistance of the well seen from the        seabed/wellhead, again derived from casing and tubular        dimensions, weights, and material type (resistivity) and        resistivity of the overburden (medium surrounding the well) but        this time for the whole completion.

In, particular examples of the above systems, the cable or cables 41used in connecting the harvesting module to the structure/surroundingsmay have a cross-sectional area of say 10 mm² to 140 mm². 10 mm² mightbe considered a low end of a desired operational cable size. Largercross-sectional area would normally be preferable. A 140 mm² cable mightbe Kerite(®) LTF3 flat type cable. This represents the upper end of whatis currently commercially available, but, if available, larger sizes canbe used.

FIG. 8 is a flow chart showing a process for optimising the energyharvesting of a harvesting module of the type described above.

In step 801 the dc to dc convertor 44 initiates using initialsettings/configuration and delivers available energy to the chargestorage means 45.

In step 802 a determination is made as to whether there is sufficientvoltage to power the microprocessor in the central unit 46. If no, thisstep 802 repeats until the answer is yes and when the answer is yes, theprocess proceeds to step 803 where the microprocessor in the centralunit 46 is powered.

Then in step 804 the microprocessor measures the power output from theenergy harvester and in step 805 the microprocessor modifies the dc todc convertor 44 settings to slightly increase load. Subsequently in step806, a determination is made as to whether this leads to an increase inharvester output. If the answer is yes then the process returns tobefore step 805 so that the dc to dc convertor 44 settings can bealtered again to slightly increase load.

On the other hand if the determination is made in step 806 that outputwas not increased then the process proceeds to step 807 where themicroprocessor modifies the dc to dc convertor 44 settings to slightlydecrease the load and the process returns to before step 806 so it canbe determined whether this has resulted in an increase in output.

After this, steps 805, 806 and 807 are repeated iteratively duringenergy harvesting such that the load is successively incremented anddecremented based on the result in step 806. Thus this leads to dynamicoptimisation of power harvesting.

As mentioned above where the dc to dc convertor 44 makes use of a FieldEffect Transistor and an accompanying transformer the step of changingthe dc to dc convertor settings in steps 805 and 807 may comprise thestep of changing the tapping used on the secondary transformer in orderto modify the load appropriately. This will also be true where such avariable transformer is provided with a H-bridge as shown in FIG. 2D.Alternatively in such a case the duty cycle of the transistors in theH-bridge may be adjusted to vary the load.

FIG. 9 shows a flow chart illustrating operation of a downhole unit 4 aof the type described above.

In step 901 it is determined whether there is sufficient power to powerthe processor in the central unit 46. If not the process stays at thisstep until there is sufficient power.

When there is sufficient power, the process proceeds to step 902 whereit is determined whether a command has been received or there is arequirement to send a scheduled set of data. If not then the processremains in this state of determining whether any action is requireduntil action is required.

When action is required, the process proceeds to step 903 where data isrecovered from a sensor or from memory as required and the loadpresented by the energy harvester module between the connections 41 a ismodulated to encode data.

Separately at the wellhead, in step 904, the voltage potential of thewell head is monitored and data is decoded in a second microprocessor.Then in step 905 the extracted data may be exported or retransmitted toa client e.g. through a seawater acoustic link or an umbilical link.

FIG. 10 shows a well installation including a platform 1000. The wellhead 1 is provided on a deck 1001 of the platform 1000. In this case themetallic structure includes a riser 1002 between the mudline and thedeck 1001. The production tubing 21 runs within the riser 1002 as wellas downhole. Casing 22, 23, is provided downhole. The innermost casing22 is a continuation of the riser 1002. Cathodic protection anodes 3Bare provided on the platform structure 1000. Electrical connection willexist between the platform and the downhole structure 2 (casing andproduction tubing). This may be via a drilling template 1003 and/or viathe well head, riser and other components such as riser guides. In suchcases it can be difficult to know where to make the upper connection ofa harvesting arrangement of the type shown in FIG. 1, 3, 4 or 6 to gainbest performance. It will not always be known where the cathodicprotection current will be injected in to the conductive pipe (the runsof elongate members) which run down into the well. As mentioned above itcan be desirable to make the upper connection adjacent the locationwhere the CP current is injected. If one is looking for optimisation,one option is to control this injection point—i.e. ensure galvanicconnection at a known point. Another option is to provide the systemwith a plurality of alternative upper connection points for theharvesting module and allow selection of the most effective connectionpoint after installation. Typically in such a case, the power deliverysystem will be installed with a plurality of upper cable connections tothe metallic structure and the best performing one selected, by, forexample, operation of a switch under control of the central unit.

Signal, Device and Sensor Options

Various particular signalling techniques are described above. For theavoidance of doubt it should be noted that a wide range of signallingtechniques may be used alone or in combination in various parts of thesignal channel in systems of the current type. Thus wireless signals maybe transmitted in at least one of the following forms: electromagnetic,acoustic, inductively coupled tubulars and coded pressure pulsing andreferences herein to “wireless”, relate to said forms, unless wherestated otherwise.

Signals, unless otherwise stated can include control and data signals.Control signals can control downhole devices including sensors. Datafrom sensors may be transmitted in response to a control signal.Moreover data acquisition and/or transmission parameters, such asacquisition and/or transmission rate or resolution, may be varied usingsuitable control signals.

Pressure pulses include methods of communicating from/to within thewell/borehole, from/to at least one of a further location within thewell/borehole, and the surface of the well/borehole, using positiveand/or negative pressure changes, and/or flow rate changes of a fluid ina tubular and/or annular space.

Coded pressure pulses are such pressure pulses where a modulation schemehas been used to encode commands and/or data within the pressure or flowrate variations and a transducer is used within the well/borehole todetect and/or generate the variations, and/or an electronic system isused within the well/borehole to encode and/or decode commands and/orthe data. Therefore, pressure pulses used with an in-well/boreholeelectronic interface are herein defined as coded pressure pulses. Anadvantage of coded pressure pulses, as defined herein, is that they canbe sent to electronic interfaces and may provide greater transmissionrate and/or bandwidth than pressure pulses sent to mechanicalinterfaces.

Where coded pressure pulses are used to transmit control signals,various modulation schemes may be used to encode control signals such asa pressure change or rate of pressure change, on/off keyed (00K), pulseposition modulation (PPM), pulse width modulation (PWM), frequency shiftkeying (FSK), pressure shift keying (PSK), amplitude shift keying (ASK),combinations of modulation schemes may also be used, for example,OOK-PPM-PWM. Transmission rates for coded pressure modulation schemesare generally low, typically less than 10 bps, and may be less than 0.1bps. Coded pressure pulses can be induced in static or flowing fluidsand may be detected by directly or indirectly measuring changes inpressure and/or flow rate. Fluids include liquids, gasses and multiphasefluids, and may be static control fluids, and/or fluids being producedfrom or injected in to the well.

Wireless signals may be such that they are capable of passing through abarrier, such as a plug or said annular sealing device, when fixed inplace.

Therefore wireless signals may be transmitted in at least one of thefollowing forms: electromagnetic, acoustic, and inductively coupledtubulars.

EM/Acoustic and coded pressure pulsing use the well, borehole orformation as the medium of transmission. The EM/acoustic or pressuresignal may be sent from the well, or from the surface. If provided inthe well, an EM/acoustic signal may be able to travel through anyannular sealing device, although for certain embodiments, it may travelindirectly, for example around any annular sealing device.

Electromagnetic and acoustic signals are useful as they can transmitthrough/past an annular sealing device without special inductivelycoupled tubulars infrastructure, and for data transmission, the amountof information that can be transmitted is normally higher compared tocoded pressure pulsing, especially receiving data from the well.

Where inductively coupled tubulars are used, there are normally at leastten, usually many more, individual lengths of inductively coupledtubular which are joined together in use, to form a string ofinductively coupled tubulars. They have an integral wire and may beformed tubulars such as tubing, drill pipe, or casing. At eachconnection between adjacent lengths there is an inductive coupling. Theinductively coupled tubulars that may be used can be provided by N O Vunder the brand Intellipipe®.

Thus, EM/acoustic or pressure wireless signals can be conveyed arelatively long distance as wireless signals, sent for at least 200 m,optionally more than 400 m or longer which is a clear benefit over othershort range signals. Inductively coupled tubulars provide thisadvantage/effect by the combination of the integral wire and theinductive couplings. The distance travelled may be much longer,depending on the length of the well.

Data and commands within signals may be relayed or transmitted by othermeans. Thus the wireless signals could be converted to other types ofwireless or wired signals, and optionally relayed, by the same or byother means, such as hydraulic, electrical and fibre optic lines. Forexample signals may be transmitted through a cable for a first distance,such as over 400 m, and then transmitted via acoustic or EMcommunications for a smaller distance, such as 200 m. In another examplethey may be transmitted for 500 m using coded pressure pulsing and then1000 m using a hydraulic line.

Non-wireless means may be used to transmit the signal in addition to thewireless means. The distance travelled by signals is dependent on thedepth of the well, often the wireless signal, including repeaters butnot including any non-wireless transmission, travel for more than 1000 mor more than 2000 m.

Different wireless signals may be used in the same well forcommunications going from the well towards the surface, and forcommunications going from the surface into the well.

Wireless signals may be sent to a communication device, directly orindirectly, for example making use of in-well relays above and/or belowany annular sealing device. A wireless signal may be sent from thesurface or from a wireline/coiled tubing (or tractor) run probe at anypoint in the well optionally above any annular sealing device.

Acoustic signals and communication may include transmission throughvibration of the structure of the well including tubulars, casing,liner, drill pipe, drill collars, tubing, coil tubing, sucker rod,downhole tools; transmission via fluid (including through gas),including transmission through fluids in uncased sections of the well,within tubulars, and within annular spaces; transmission through staticor flowing fluids; mechanical transmission through wireline, slicklineor coiled rod; transmission through the earth; transmission throughwellhead equipment. Communication through the structure and/or throughthe fluid are preferred.

Acoustic transmission may be at sub-sonic (<20 Hz), sonic (20 Hz-20kHz), and ultrasonic frequencies (20 kHz-2 MHz). Preferably the acoustictransmission is sonic (20 Hz-20 khz).

Acoustic signals and communications may include Frequency Shift Keying(FSK) and/or Phase Shift Keying (PSK) modulation methods, and/or moreadvanced derivatives of these methods, such as Quadrature Phase ShiftKeying (QPSK) or Quadrature Amplitude Modulation (QAM), and preferablyincorporating Spread Spectrum Techniques. Typically they are adapted toautomatically tune acoustic signalling frequencies and methods to suitwell conditions.

Acoustic signals and communications may be uni-directional orbi-directional. Piezoelectric, moving coil transducer ormagnetostrictive transducers may be used to send and/or receive thesignal.

Electromagnetic (EM) (sometimes referred to as Quasi-Static (QS))wireless communication is normally in the frequency bands of: (selectedbased on propagation characteristics)

-   -   sub-ELF (extremely low frequency) <3 Hz (normally above 0.01        Hz);    -   ELF 3 Hz to 30 Hz;    -   SLF(super low frequency) 30 Hz to 300 Hz;    -   ULF (ultra low frequency) 300 Hz to 3 kHz; and,    -   VLF (very low frequency) 3 kHz to 30 kHz.

An exception to the above frequencies is EM communication using the pipeas a wave guide, particularly, but not exclusively when the pipe is gasfilled, in which case frequencies from 30 kHz to 30 GHz may typically beused dependent on the pipe size, the fluid in the pipe, and the range ofcommunication. The fluid in the pipe is preferably non-conductive. U.S.Pat. No. 5,831,549 describes a telemetry system involving gigahertztransmission in a gas filled tubular waveguide.

Sub-ELF and/or ELF are useful for communications from a well to thesurface (e.g. over a distance of above 100 m). For more localcommunications, for example less than 10 m, VLF is useful. Thenomenclature used for these ranges is defined by the InternationalTelecommunication Union (ITU). EM communications may includetransmitting communication by one or more of the following: imposing amodulated current on an elongate member and using the earth as return;transmitting current in one tubular and providing a return path in asecond tubular; use of a second well as part of a current path;near-field or far-field transmission; creating a current loop within aportion of the well metalwork in order to create a potential differencebetween the metalwork and earth; use of spaced contacts to create anelectric dipole transmitter; use of a toroidal transformer to imposecurrent in the well metalwork; use of an insulating sub; a coil antennato create a modulated time varying magnetic field for local or throughformation transmission; transmission within the well casing; use of theelongate member and earth as a coaxial transmission line; use of atubular as a wave guide; transmission outwith the well casing.

Especially useful is imposing a modulated current on an elongate memberand using the earth as return; creating a current loop within a portionof the well metalwork in order to create a potential difference betweenthe metalwork and earth; use of spaced contacts to create an electricdipole transmitter; and use of a toroidal transformer to impose currentin the well metalwork.

To control and direct current advantageously, a number of differenttechniques may be used. For example one or more of: use of an insulatingcoating or spacers on well tubulars; selection of well control fluids orcements within or outwith tubulars to electrically conduct with orinsulate tubulars; use of a toroid of high magnetic permeability tocreate inductance and hence an impedance; use of an insulated wire,cable or insulated elongate conductor for part of the transmission pathor antenna; use of a tubular as a circular waveguide, using SHF (3 GHzto 30 GHz) and UHF (300 MHz to 3 GHz) frequency bands.

Various means for receiving a transmitted signal can be used, these mayinclude detection of a current flow; detection of a potentialdifference; use of a dipole antenna; use of a coil antenna; use of atoroidal transformer; use of a Hall effect or similar magnetic fielddetector; use of sections of the well metalwork as part of a dipoleantenna.

Where the phrase “elongate member” is used, for the purposes of EMtransmission, this could also mean any elongate electrical conductorincluding: liner; casing; tubing or tubular; coil tubing; sucker rod;wireline; drill pipe; slickline or coiled rod.

Gauges can comprise one or more of various different types of sensor.The or each sensor can be coupled (physically or wirelessly) to awireless transmitter and data can be transmitted from the wirelesstransmitter to above the annular sealing device or otherwise towards thesurface. Data can be transmitted in at least one of the following forms:electromagnetic, acoustic and inductively coupled tubulars, especiallyacoustic and/or electromagnetic as described herein above.

Such short range wireless coupling may be facilitated by EMcommunication in the VLF range.

The sensors provided may sense any parameter and so be any type ofsensor including but not necessarily limited to, such as temperature,acceleration, vibration, torque, movement, motion, cement integrity,pressure, direction and inclination, load, various tubular/casingangles, corrosion and erosion, radiation, noise, magnetism, seismicmovements, stresses and strains on tubular/casings including twisting,shearing, compressions, expansion, buckling and any form of deformation;chemical or radioactive tracer detection; fluid identification such asgas detection; water detection, carbon dioxide detection, hydrate, waxand sand production; and fluid properties such as (but not limited to)flow, density, water cut, resistivity, pH, viscosity, bubble point,gas/oil ratio, hydrocarbon composition, fluid colour or fluorescence.The sensors may be imaging, mapping and/or scanning devices such as, butnot limited to, camera, video, infra-red, magnetic resonance, acoustic,ultra-sound, electrical, optical, impedance and capacitance. Sensors mayalso monitor equipment in the well, for example valve position, or motorrotation. Furthermore the sensors may be adapted to induce the signal orparameter detected by the incorporation of suitable transmitters andmechanisms.

The apparatus especially the sensors, may comprise a memory device whichcan store data for recovery at a later time. The memory device may also,in certain circumstances, be retrieved and data recovered afterretrieval. The memory device may be configured to store information forat least one minute, optionally at least one hour, more optionally atleast one week, preferably at least one month, more preferably at leastone year or more than five years.

1. Downhole data communication apparatus for use in a well installationhaving metallic structure provided with a cathodic protection systemsuch that there is an electrical circuit comprising the metallicstructure and an earth return around which an electrical current flowsas a result of the cathodic protection system, the downhole datacommunication apparatus comprising: a first communication module forlocation at a first location and comprising modulation means formodulating the electrical current at a first location so as to encodedata; and a second communication module for location at a secondlocation, spaced from the first location, and comprising a detector fordetecting the effect of the modulation of the electrical current at thefirst location so as to extract said data.
 2. Downhole datacommunication apparatus according to claim 1 in which the modulationmeans is arranged to at least one of: i) where the cathodic protectionsystem is an impressed cathodic protection system, control a signalsource of the impressed cathodic protection system to directly modulatethe cathodic protection current applied to the metallic structure; ii)modify the connection between at least one anode of the cathodicprotection system and the metallic structure; and iii) alter theimpedance of the electrical circuit.
 3. Downhole data communicationapparatus according to claim 1 wherein the first communication module isarranged for location downhole.
 4. Downhole data communication apparatusaccording to claim 1 wherein the second communication module is arrangedfor location downhole.
 5. Downhole data communication apparatusaccording to claim 1 comprising a sensor module for sensing at least oneparameter, wherein the first communication module is arranged forsending data encoding readings from the sensor module towards the secondcommunication module.
 6. Downhole data communication apparatus accordingto claim 5 wherein the sensor module comprises a pressure sensor. 7.Downhole data communication apparatus according to claim 1 wherein thesecond communication module is arranged for providing data to a downholedevice in dependence on data received by the second communication modulefrom the first communication module.
 8. Downhole data communicationapparatus according to claim 7, wherein the downhole device comprises atleast one of: a downhole sensor; a downhole actuator; an annular sealingdevice, for example a packer, or a packer element; a valve; a downholecommunication module, for example a transceiver or repeater.
 9. Downholedata communication apparatus according to claim 8, wherein the valvecomprises at least one of: a subsurface safety valve; a bore flowcontrol valve; a bore to annulus valve; an annulus to annulus valve; abore to pressure compensation chamber valve; an annulus to pressurecompensation chamber valve; a through packer or packer bypass valve. 10.Downhole data communication apparatus according to claim 1 wherein atleast one of the first and second communication modules comprises acommunications repeater for location downhole in a well and arranged forcommunicating with a first device beyond the well head using acommunication channel which is wireless at least through the well headand arranged for communicating with second device located in the welland thus below the well head such that the communications repeater mayact as a repeater between the first and second devices.
 11. Downholedata communication apparatus according to claim 1 comprising a downholeelectrical power harvesting module arranged for electrical connectionbetween two spaced locations in a well installation and comprising anelectrical circuit arranged for harvesting electrical energy, in use,from a potential difference between the spaced locations, used forharvesting, which acts as an input voltage, the harvesting module beingarranged for supplying power to at least one component of thecommunication apparatus.
 12. Downhole data communication apparatusaccording to claim 11 in which the first communication module isarranged for controlling the load generated by the harvesting module tocause said modulation of the electric current in the metallic structureat the signalling location.
 13. Downhole data communication apparatusaccording to claim 11 wherein the harvesting module is arranged toharvest electrical energy from dc currents.
 14. A downhole datacommunication system comprising downhole data communication apparatusaccording to claim 1 located in a well installation having metallicstructure provided with a cathodic protection.
 15. A downhole datacommunication system for use in a well installation having metallicstructure provided with a cathodic protection system such that there isan electrical circuit comprising the metallic structure and an earthreturn around which an electrical current flows as a result of thecathodic protection system, the system comprising downhole datacommunication apparatus comprising: a first communication module locatedat a first location and comprising modulation means for modulating theelectrical current at the first location so as to encode data; and asecond communication module located at a second location, spaced fromthe first location, and comprising a detector for detecting the effectof the modulation of the electrical current at the first location so asto extract said data.
 16. A downhole data communication system accordingto claim 14, wherein the well is a subsea well.
 17. A downhole datacommunication system according to claim 14 wherein the apparatuscomprises a downhole electrical power harvesting module electricallyconnected between two spaced locations in the well installation andcomprising an electrical circuit arranged for harvesting electricalenergy, in use, from a potential difference between the spacedlocations, used for harvesting, which acts as an input voltage, theharvesting module being arranged for supplying power to at least onecomponent of the communication apparatus.
 18. A downhole datacommunication system according to claim 17 wherein the current flowwithin portions of the metallic structure in regions between the spacedlocations, used for harvesting, is in the same longitudinal direction.19. A downhole data communication system according to claim 17 whereinthere is an uninterrupted current flow path between the spacedlocations, used for harvesting, which is at least partly via themetallic structure.
 20. A downhole data communication system accordingto claim 14, wherein at least one of the first communication module andthe second communication module is located in an enclosed annulus of thewell.
 21. A downhole data communication system according to claim 14comprising a pressure sensor arranged for monitoring the reservoirpressure of the well.
 22. A downhole data communication system accordingto claim 14 comprising a pressure sensor arranged for monitoring thepressure in an annulus of the well.
 23. A downhole data communicationsystem according to claim 14 comprising a pressure sensor arranged formonitoring the pressure in an enclosed annulus of the well.